Motivation

It’s clear that a lot of programmers are uncomfortable with exceptions [1] [2]; in the feedback of an article I wrote about casting, it seemed that many programmers saw the throwing of a NullReferenceException at a cast to be an incredible catastrophe.

In this article, I’ll share a philosophy that I hope will help programmers overcome the widespread fear of exceptions. It’s motivated by five goals:

1. Do no harm
2. To write as little error handling code as possible,
3. To think about error handling as little as possible
4. To handle errors should handled correctly when possible,
5. Otherwise errors should be handled sanely

To do that, I

1. Use finally to stabilize program state when exceptions are thrown
2. Catch and handle exceptions locally when the effects of the error are local and completely understood
3. Wrap independent units of work in try-catch blocks to handle errors that have global impact

This isn’t the last word on error handling, but it avoids many of the pitfalls that people fall into with exceptions. By building upon this strategy, I believe it’s possible to develop an effective error handling strategy for most applications: future articles will build on this topic, so keep posted by subscribing to the Generation 5 RSS Feed.

The Tragedy of Checked Exceptions

Java’s done a lot of good, but checked exceptions are probably the worst legacy that Java has left us. Java has influenced Python, PHP, Javascript, C# and many of the popular languages that we use today. Unfortunately, checked exceptions taught Java programmers to catch exceptions prematurely, a habit that Java programmers carried into other languages, and has result in a code base that sets bad examples.

Most exceptions in Java are checked, which means that the compiler will give you an error if you write

[01] public void myMethod() {
[02]    throw new ItDidntWorkException()
[03] };

unless you either catch the exception inside myMethod or you replace line [01] with

[04] public void myMethod() throws ItDidntWorkException {

The compiler is also aware of any checked exceptions that are thrown by methods underneath myMethod, and forces you to either catch them inside myMethod or to declare them in the throws clause of myMethod.

I thought that this was a great idea when I started programming Java in 1995. With the hindsight of a decade, we can see that it’s a disaster. The trouble is that every time you call a method that throws an exception, you create an immediate crisis: you break the build. Rather than conciously planning an error handling strategy, programmers do something, anything, to make the compiler shut up. Very often you see people bypass exceptions entirely, like this:

[05] public void someMethod() {
[06]    try {
[07]       objectA.anotherMethod();
[08]    } catch(SubsystemAScrewedUpException ex) { };
[09] }

Often you get this instead:

[10]    try {
[11]       objectA.anotherMethod();
[12]    } catch(SubsystemAScrewedUp ex) {
[13]        // something ill-conceived to keep the compiler happy
[14]    }
[15]    // Meanwhile,  the programmer makes a mistake here because
[16]    // writing an exception handler broke his concentration

This violates the first principle, to “do no harm.” It’s simple, and often correct, to pass the exception up to the calling function,

[17] public int someMethod() throws SubsystemAScrewedUp {

But, this still breaks the build, because now every function that calls someMethod() needs to do something about the exception. Imagine a program of some complexity that’s maintained by a few programmers, in which method A() calls method B() which calls method C() all the way to method F().

The programmer who works on method F() can change the signature of that method, but he may not have the authority to change the signature of the methods above. Depending on the culture of the shop, he might be able to do it himself, he might talk about it with the other programmers over IM, he might need to get the signatures of three managers, or he might need to wait until the next group meeting. If they keep doing this, however, A() might end up with a signature like

[18] public String A() throws SubsystemAScrewedUp, IOException, SubsystemBScrewedUp,
[19]   WhateverExceptionVendorZCreatedToWrapANullPointerException, ...

This is getting out of hand, and they realize they can save themselvesa lot of suffering by just writing

[20] public int someMethod() throws Exception {

all the time, which is essentially disabling the checked exception mechanism. Let’s not dismiss the behavior of the compiler out of hand, however, because it’s teaching us an important lesson: error handling is a holistic property of a program: an error handled in method F() can have implications for methods A()…E(). Errors often break the assumption of encapsulation, and require a global strategy that’s applied consistently throughout the code.

PHP, C# and other post-Java languages tend to not support checked exceptions. Unfortunately, checked exception thinking has warped other langages, so you’ll find that catch statements are used neurotically everywhere.

Exception To The Rule: Handling Exceptions Locally

Sometimes you can anticipate an exception, and know what exact action to take. Consider the case of a compiler, or a program that processes a web form, which might find more than one error in user input. Imagine something like (in C#):

[21] List<string> errors=new List<string>();
[22] uint quantity=0;
[23] ... other data fields ...
[24]
[25] try {
[26]   quantity=UInt32.Parse(Params["Quantity"]);
[27] } catch(Exception ex) {
[28]   errors.Add("You must enter a valid quantity");
[29] }
[30]
[31] ... other parsers/validators ...
[32]
[33] if (errors.Empty()) {
[34]    ... update database,  display success page ...
[35] } else {
[36]    ... redraw form with error messages ...
[37] }

Here it makes sense to catch the exception locally, because the exceptions that can happen on line [22] are completely handled, and don’t have an effect on other parts of the application. The one complaint you might make is that I should be catching something more specific than Exception. Well, that would bulk the code up considerably and violate the DRY (don’t repeat yourself) principle: UInt32.Parse can throw three different exceptions: ArgumentNullException, FormatException, and OverflowException. On paper, the process of looking up the “Quantity” key in Params could throw an ArgumentNullException or a KeyNotFoundException.

I don’t think either ArgumentNullException can really happen, and I think the KeyNotFoundException would only occur in development, or if somebody was trying to submit the HTML form with an unauthorized program. Probably the best thing to do in either case would be to abort the script with a 500 error and log the details, but the error handling on line [24] is sane in that it prevents corruption of the database.

The handling of FormatException and OverflowException, in the other case, is fully correct. The user gets an error message that tells them what they need to do to fix the situation.

This example demonstrates a bit of why error handling is so difficult and why the perfect can be the enemy of the good: the real cause of an IOException could be a microscopic scratch on the surface of a hard drive, and operating system error, or the fact that somebody spilled a coke on a router in Detroit — diagnosing the problem and offering the right solution is an insoluble problem.

Fixing it up with finally

The first exception handling construct that should be on your fingertips is finally, not catch. Unfortunately, finally is a bit obscure: the pattern in most languages is

[38] try {
[39]    ... do something that might throw an exception ...
[40] } finally {
[41]    ... clean up ...
[42] }

The code in the finally clause get runs whether or not an exception is thrown in the try block. Finally is a great place to release resources, roll back transactions, and otherwise protect the state of the application by enforcing invariants. Let’s think back to the chain of methods A() through F(): with finally, the maintainer of B() can implement a local solution to a global problem that starts in F(): no matter what goes wrong downstream, B() can repair invariants and repair the damage. For instance, if B()’s job is to write something into a transactional data store, B() can do something like:

[43] Transaction tx=new Transaction();
[44] try {
[45]    ...
[46]    C();
[47]    ...
[48]    tx.Commit();
[49] } finally {
[50]    if (tx.Open)
[51]        tx.Rollback();
[52] }

This lets the maintainer of B() act defensively, and offer the guarantee that the persistent data store won’t get corrupted because of an exception that was thrown in the try block. Because B() isn’t catching the exception, it can do this without depriving upstream methods, such as A() from doing the same.

C# gets extra points because it has syntactic sugar that makes a simple case simple: The using directive accepts an IDisposable as an argument and wraps the block after it with a finally clause that calls the Dispose() method of the IDisposable. ASP.NET applications can fail catastrophically if you don’t Dispose() database connections and result sets, so

[53] using (var reader=sqlCommand.ExecuteReader()) {
[54]   ... scroll through result set ...
[55] }

is a widespread and effective pattern.

PHP loses points because it doesn’t support finally. Granted, finally isn’t as important in PHP, because all resources are released when a PHP script ends. The absense of finally, however, encourages PHP programmers to overuse catch, which perpetuates exception phobia. The PHP developers are adding great features to PHP 5.3, such as late static binding, so we can hope that they’ll change their mind and bring us a finally clause.

Where should you catch exceptions?

At high levels of your code, you should wrap units of work in a try-catch block. A unit of work is something that makes sense to either give up on or retry. Let’s work out a few simple examples:

Scripty Command line program: This program is going to be used predominantly by the person who wrote it and close associates, so it’s acceptable for the program to print a stack trace if it fails. The “unit of work” is the whole program.

Command line script that processes a million records: It’s likely that some records are corrupted or may trigger bugs in the program. Here it’s reasonable for the “unit of work” to be the processing of a single record. Records that cause exceptions should be logged, together with a stack trace of the exception.

Web application: For a typical web application in PHP, JSP or ASP.NET, the “unit of work” is the web request. Ideally the application returns a “500 Internal Error”, displays a message to the user (that’s useful but not overly revealing) and logs the stack trace (and possibly other information) so the problem can be investigated. If the application is in debugging mode, it’s sane to display the stack trace to the web browser.

GUI application: The “unit of work” is most often an event handler that’s called by the GUI framework. You push a button, something does wrong, then what? Unlike server-side web applications, which tend to assume that exceptions don’t involve corruption of static memory or of a database, GUI applications tend to shut down when they experience unexpected exceptions. [3] As a result, GUI applications tend to need infrastructure to convert common and predictable exceptions (such as network timeouts) into human readable error messages.

Mail server: A mail server stores messages in a queue and delivers them over a unreliable network. Exceptions occur because of full disks (locally or remote), network failures, DNS misconfigurations, remote server falures, and an occasionaly cosmic ray. The “unit of work” is the delivery of a single message. If an exception is thrown during delivery of the message, it stays in the queue: the mail server attempts to resend on a schedule, discarding it if it is unable to deliver after seven days.

What should you do when you’ve caught one?

That’s the subject of another article. Subscribe to my RSS feed if you want to read it when it’s ready. For now, I’ll enumerate a few questions to think about:

1. What do tell the end user?
2. What do you tell the developer?
3. What do you tell the sysadmin?
4. Will the error clear if up if we try to repeat this unit of work again?
5. How long would we need to wait?
6. Could we do something else instead?
7. Did the error happen because the state of the application is corrupted?
8. Did the error cause the state of the application to get corrupted?

Conclusion

Error handling is tough. Because errors come from many sources such as software defects, bad user input, configuration mistakes, and both permanent and transient hardware failures, it’s impossible for a developer to anticipate and perfectly handle everything that can go wrong. Exceptions are an excellent method of separating error handling logic from the normal flow of programs, but many programmers are too eager to catch exceptions: this either causes errors to be ignores, or entangles error handling with mainline logic, complicating both. The long term impact is that many programmers are afraid of exceptions and turn to return values as an error signals, which is a step backwards.

A strategy that (i) uses finally as the first resort for containing corrupting and maintaining invariants, (ii) uses catch locally when the exceptions thrown in an area are completely understood, and (iii) surrounds independent units of work with try-catch blocks is an effective basis for using exceptions that can be built upon to develop an exception handling policy for a particular application.

Error handling is a topic that I spend entirely too much time thinking about, so I’ll be writing about it more. Subscribe to my RSS Feed if you think I’ve got something worthwhile to say.

Introduction

The first language I used that put dictionaries on my fingertips was Perl, where the solution to just about any problem involved writing something like

$hashtable{$key}=$value;

Perl called a dictionary a ‘hash’,  a reference to the way Perl implemented dictionaries.  (Dictionaries are commonly implemented with hashtables and b-trees,  but can also be implemented with linked-list and other structures.)  The syntax of Perl is a bit odd, as you’d need to use $, # or % to reference scalar,  array or hash variables in different contexts,  but dictionaries with similar semantics became widespread in dynamic languages of that and succeeding generations, such as Python, PHP and Ruby.  ‘Map’ container classes were introduced in Java about a decade ago,  and programmers are using dictionaries increasingly in static languages such as Java and C#.

Dictionaries are a convenient and efficient data structure, but there’s are areas in which different mplementations behave differently: for instance,  in what happens if you try to access an undefined key.   I think that cross-training is good for developers,  so this article compares this aspect of the semantics of dictionaries in four popular languages:  PHP,  Python,  Java and C#.

Use cases

There are two use cases for dictionaries, so far as error handling is concerned:

1. When you expect to look up undefined values, and
2. When you don’t

Let’s look at three examples:

Computing A Histogram

One common use for a dictionary is for counting items, or recording that items in a list or stream have been seen. In C#, this is typically written something like:

[01] var count=Dictionary<int,int>();
[02] foreach(int i in inputList) {
[03]   if (!counts.Contains(i))
[04]       count[i]=0;
[05]
[06]   count[i]=count[i]+1
[07] }

The Dictionary count now contains the frequency of items inputList, which could be useful for plotting a histogram. A similar pattern can be used if we wish to make a list of unique items found in inputList. In either case,  looking up values that aren’t already in the hash is a fundamental part of the algorithm.

Processing Input

Sometimes, we’re getting input from another subsystem, and expect that some values might not be defined. For instance, suppose a web site has a search feature with a number of optional features, and that queries are made by GET requests like:

[08] search.php?q=kestrel
[09] search.php?q=admiral&page=5
[10] search.php?q=laurie+anderson&page=3&in_category=music&after_date=1985-02-07

In this case, the only required search parameter is “q”, the query string — the rest are optional. In PHP (like many other environments), you can get at GET variables via a hashtable, specifically, the $_GET superglobal, so (depending on how strict the error handling settings in your runtime are) you might write something like

[11] if ($_GET["q"])) {
[12]     throw new InvalidInputException("You must specify a query");
[13] }
[14]
[15] if($_GET["after_date"]) {
[16]  ... add another WHERE clause to a SQL query ...
[17] }

This depends, quite precisely, on two bits of sloppiness in PHP and Perl: (a) Dereferencing an undefined key on a hash returns an undefined value, which is something like a null. (b) both languages have a liberal definition of true and false in an if() statement. As a result, the code above is a bit quirky. The if() at line 11 evaluates false if q is undefined, or if q is the empty string. That’s good. However, both the numeric value 0 and the string “0″ also evaluate false. As a result, this code won’t allow a user to search for “0″, and will ignore an (invalid) after_date of 0, rather than entering the block at line [16], which hopefully would validate the date.

Java and C# developers might enjoy a moment of schadenfreude at the above example, but they’ve all seen, written and debugged examples of input handling code that just as quirky as the above PHP code — with several times the line count. To set the record straight, PHP programmers can use the isset() function to precisely test for the existence of a hash key:

[11] if (isset($_GET["q"]))) {
[12]     throw new InvalidInputException("You must specify a query");
[13] }

The unusual handling of “0″ is the kind of fault that can survive for years in production software:  so long as nobody searches for “0″,  it’s quite harmless.  (See what you get if you search for a negative integer on Google.)  The worst threat that this kind of permissive evaluation poses is when it opens the door to a security attack,  but we’ve also seen that highly complex logic that strives to be “correct” in every situation can hide vulnerabilities too.

Relatively Rigid Usage

Let’s consider a third case: passing a bundle of context in an asynchronous communications call in a Silverlight application written in C#. You can do a lot worse than to use the signatures:

[14] void BeginAsyncCall(InputType input,Dictionary<string, object> context,CallbackDelegate callback);
[15] void CallbackDelegate(ReturnType returnValue,Dictionary<string,object> context);

The point here is that the callback might need to know something about the context in which the asynchronous function was called to do it’s work. However, this information may be idiosyncratic to the particular context in which the async function is called,  and is certainly not the business of the asynchronous function. You might write something like

[16] void Initiator() {
[17]   InputType input=...;
[18]   var context=Dictionary<string,object>();
[19]   context["ContextItemOne"]= (TypeA) ...;
[20]   context["ContextItemTwo"]= (TypeB) ...;
[21]   context["ContextItemThre"] = (TypeC) ...;
[22]   BeginAsyncCall(input,context,TheCallback);
[23] }
[24]
[25] void TheCallback(ReturnType output,Dictionary<string,object> context) {
[26]   ContextItemOne = (TypeA) context["ContextItemOne"];
[27]   ContextItemTwo = (TypeB) context["ContextItemTwo"];
[28]   ContextItemThree = (TypeC) context["ContextItemThree"];
[29]   ...
[30] }

This is nice, isn’t it?  You can pass any data values you want between Initiator and TheCallback. Sure,  the compiler isn’t checking the types of your arguments,  but loose coupling is called for in some situations.  Unfortunately it’s a little too loose in this case,  because we spelled the name of a key incorrectly on line 21.

What happens?

The [] operator on a dot-net Dictionary throws a KeyNotFoundException when we try to look up a key that doesn’t exist.   I’ve set a global exception handler for my Silverlight application which,  in debugging mode,  displays the stack trace.  The error gets quickly diagnosed and fixed.

Four ways to deal with a missing value

There are four tools that hashtables give programmers to access values associated with keys and detect missing values:

1. Test if key exists
2. Throw exception if key doesn’t exist
3. Return default value (or null) if key doesn’t exist
4. TryGetValue

#1: Test if key exists

PHP:    isset($hashtable[$key])
Python: key in hashtable
C#:     hashtable.Contains(key)
Java:   hashtable.containsKey(key)

This operator can be used together with the #2 or #3 operator to safely access a hashtable.  Line [03]-[04] illustrates a common usage pattern.

One strong advantage of the explicit test is that it’s more clear to developers who spend time working in different language environments — you don’t need to remember or look in the manual to know if the language you’re working in today uses the #2 operator or the #3 operator.

Code that depends on the existence test can be more verbose than alternatives,  and can  be structurally unstable:  future edits can accidentally change the error handling properties of the code.  In multithreaded environments,  there’s a potential risk that an item can be added or removed between the existance check and an access — however,  the default collections in most environment are not thread-safe,  so you’re likely to have worse problems if a collection is being accessed concurrently.

#2 Throw exception if key doesn’t exist

Python: hashtable[key]
C#:     hashtable[key]

This is a good choice when the non-existence of a key is really an exceptional event.  In that case,  the error condition is immediately propagated via the exception handling mechanism of the language,  which,  if properly used,  is almost certainly better than anything you’ll develop.  It’s awkward,  and probably inefficient,  if you think that non-existent keys will happen frequently.  Consider the following rewrite of the code between [01]-[07]

[31] var count=Dictionary<int,int>();
[32] foreach(int i in inputList) {
[33]   int oldCount;
[34]   try {
[35]       oldCount=count[i];
[36]   } catch (KeyNotFoundException ex) {
[37]       oldCount=0
[38]   }
[39]
[40]   count[i]=oldCount+1
[41] }

It may be a matter of taste,  but I think that’s just awful.

#3 Return a default (often null) value if key doesn’t exist

PHP:    $hashtable[key] (well,  almost)
Python: hashtable.get(key, [default value])
Java:   hashtable.get(key)

This can be a convenient and compact operation.  Python’s form is particularly attractive because it lets us pick a specific default value.  If we use an extension method to add a Python-style GetValue operation in C#,  the code from [01]-[07] is simplified to

[42] var count=Dictionary<int,int>();
[43] foreach(int i in inputList)
[44]   count[i]=count.GetValue(i,0)+1;

It’s reasonable for the default default value to be null (or rather,  the default value of the type),  as it is in Python,  in which case we could use the ??-operator to write

[42] var count=Dictionary<int,int>();
[43] foreach(int i in inputList)
[44]   count[i]=(count.GetValue(i) ?? 0)+1;

(A ?? B equals A if A is not null,  otherwise it equals B.)   The price for this simplicity is two kinds of sloppiness:

1. We can’t tell the difference between a null (or default) value associated with a key and no value associated with a key
2. The potential of null value exports chaos into the environment:  trying to use a null value can cause a NullReferenceException if we don’t explictly handle the null.  NullReferenceExceptions don’t bother me if they happen locally to the function that returns them,  but they can be a bear to understand when a null gets written into an instance variable that’s accessed much later.

Often people don’t care about 1,  and the risk of 2 can be handled by specifying a non-null default value.

Note that PHP’s implementation of hashtables has a particularly annoying characteristic.  Error handling in php is influenced by the error_reporting configuration variable which can be set in the php.ini file and other places.  If the E_STRICT bit is not set in error_reporting,   PHP barrels on past places where incorrect variable names are used:

[45] $correctVariableName="some value";
[46] echo "[{$corectValiableName}]"; // s.i.c.

In that case, the script prints “[]” (treats the undefined variable as an empty string) rather than displaying an error or warning message.  PHP will give a warning message if E_STRICT is set,  but then it applies the same behavior to hashtables:  an error message is printed if you try to dereference a key that doesn’t exist — so PHP doesn’t consistently implement type #3 access.

#4 TryGetValue

There are quite a few methods (Try-* methods) in the .net framework that have a signature like this:

[47] bool Dictionary<K,V>.TryGetValue(K key,out V value);

This method has crisp and efficient semantics which could be performed in an atomic thread-safe manner:  it returns true if finds the key,  and otherwise returns false.  The output parameter value is set to the value associated with the key if a value is associated with the key,  however,  I couldn’t find a clear statement of what happens if the key isn’t found.  I did a little experiment:

[48] var d = new Dictionary<int, int>();
[49] d[1] = 5;
[50] d[2] = 7;
[51] int outValue = 99;
[52] d.TryGetValue(55, out outValue)
[53] int newValue = outValue;

I set a breakpoint on line 53 and found thate the value of outValue was 0,  which is the default value of the int type.  It seems,  therefore,  that TryGetValue returns the default value of the type when it fails to find the key.  I wouldn’t count on this behavior,  as it is undocumented.

The semantics of TryGetValue are crisp and precise.  It’s particularly nice that something like TryGetValue could be implemented as an atomic operation,  if the underyling class is threadsafe.  I fear,  however,  that TryGetValue exports chaos into it’s environment.  For instance,  I don’t like declaring a variable without an assignment,  like below:

[54] int outValue;
[55] if (d.TryGetValue(55,outValue)) {
[56] ... use outValue ...
[57] }

The variable outValue exists before the place where it’s set,  and outside of the block where it has a valid value.  It’s easy for future maintainers of the code to try to use outValue between lines [54]-[55] or after line [57].  It’s also easy to write something like 51],  where the value 99 is completely irrelevant to the program.  I like the construction

[58] if (d.Contains(key)) {
[59]    int value=d[key];
[60]    ... do something with value ...
[61] }

because the variable value only exists in the block [56]-[58] where it has a defined value.

Hacking Hashables

A comparison of hashtables in different languages isn’t just academic.  If you don’t like the operations that your language gives you for hashtables,  you’re free to implement new operations.  Let’s take two simple examples.  It’s nice to have a Python-style get() in PHP that never gives a warning message,  and it’s easy to implement

[62] function array_get($array,$key,$defaultValue=false) {
[63]   if (!isset($array[$key]))
[64]      return $defaultValue;
[65]
[66]   return $array[$key];
[67] }

Note that the third parameter of this function uses a default value of false,  so it’s possible to call it in a two-parameter form

[68] $value=array_get($array,$key);

with a default default of false,  which is reasonable in PHP.

Extension methods make it easy to add a Python-style get() to C#;  I’m going to call it GetValue() to be consistent with TryGetValue():

[69] public static class DictionaryExtensions {
[70]   public static V GetValue<K, V>(this IDictionary<K, V> dict, K key) {
[71]      return dict.GetValue(key, default(V));
[72]   }
[73]
[74]   public static V GetValue<K, V>(this IDictionary<K, V> dict, K key, V defaultValue) {
[75]      V value;
[76]      return dict.TryGetValue(key, out value) ? value : defaultValue;
[77]   }
[78] }

Conclusion

Today’s programming languages put powerful data structures,  such as dictionaries,  on your fingertips.  When we look closely,  we see subtle differences in the APIs used access dictionaries in different languages.  A study of the different APIs and their consequences can help us think about how to write code that is more reliable and maintainable,  and informs API design in every language

Introduction

Extension methods are the most controversial feature that Microsoft has introduced in C# 3.0.  Introduced to support the LINQ query framework,  extension methods make it possible to define new methods for existing classes.

Although extension methods can greatly simplify code that uses them,  many are concerned that they could transform C# into something that programmers find unrecognizable,  or that C#’s namespace mechanisms are inadequate for managing large systems that use extension methods.  Adoption of the LINQ framework,  however,  means that extension methods are here to stay,  and that .net programmers need to understand how to use them effectively,  and,  in particular,  how extension methods are different from regular methods.

This article discusses three ways in which extension methods differ from regular methods:

1. Extension methods can be called on null objects without throwing an exception
2. Extension methods cannot be called inside of a subclass without the use of ‘this’
3. The precedence rules for extension methods

The Menace of Null Values

The treatment of null values is one of the major weaknesses of today’s generation of languages.  Although C# makes it possible to make nullable versions of value types,  such as int? and guid?,  there’s nothing in C# or Java like the “NOT NULL” declaration in SQL.  As a result,  handling nulls is a significant burden to writing correct code.  Consider the simple case where you want to write

[01] someObject.DoSomething();

(where DoSomething is an ordinary instance method)  When I type something like this,  Resharper often highlights the line of code to warn me that someObject might be null.  In some cases I might be confident that it never will,  but if there is any change that it will be null,  I’ll need to write something like

[02] if(someObject!=null) {
[03]    someObject.DoSomething();
[04] }

or maybe

[05] if(someObject==null)
[06]    return;
[07] someObject.DoSomething();

Alternatively I could accepted that an exception could be thrown by the invocation and decide to catch it (or not catch it) elsewhere in the application.  In two cases out of three,  one line of code gets bulked up to three.  Worse than that,  I need to make a decision at that point about what to when there’s an error condition — each decision is a case where somebody can make the wrong decision.  Even if coders make the wrong decision 5% of the time,  that would be 50 time bombs in your code for every 1000 method invocations.  (Oliver Steele works out a particularly outrageous but common case where it takes 10 lines of null-checking code to protect 1 line of working code.)

Extension Methods Can Accept Null Values

What does this have to do with extension methods?

Unlike ordinary instance methods,  extension methods do not automatically throw an exception if you call them on a null-valued object.  Depending on your point of view,  this can be (i) a gotcha,  or (ii) a useful tool for simplifying your code.  Here’s a little example:

[08] namespace ExtensionMethodTest {
[09]
[10]   static class ObjectExtension {
[11]       static public bool IsNull(this object o) {
[12]            return o == null;
[13]       }
[14]    }
[15]
[16]    class Program {
[17]        static void Main(string[] args) {
[18]            String s1 = "hello";
[19]            Console.WriteLine(s1.IsNull());
[20]            String s2 = null;
[21]            Console.WriteLine(s2.IsNull());
[22]            Console.WriteLine(s2.ToUpper());
[23]        }
[24]    }
[25] }

This example does something a bit bold:  it attaches an extension method to object,   adding an extenson method to every object in the system.  This method,  object.IsNull() returns true if object is null and false if it isn’t.  Some people might see this as a nice example of syntactic sugar,  others may see it as reckless.  What’s important is that it works:  if you run this program from the command line,  line [21] will print ‘true’,  while line [22],  which uses an ordinary method,  will throw a NullReferenceException.

Events and Extension Methods for Delegates

Chris Brandsma works out a practical example of how extension methods can be used to fix a broken and dangerous API.  That is,  the event handling mechanism commonly used in C#:

[26] public eventEventHandler<EventArgs> OnLoadData;
[27] ...
[28] OnLoadData += SomeEventHandler;
[29] ...
[30] OnLoadData(this, argument);

OnLoadData is a MulticastDelegate.  You can attach an unlimited number of real delegates to it.  The sample above works great if you attach at least one delegate,  but it fails with a NullReferenceException if you don’t.  Perhaps this isn’t a problem for you,  because you’re smart and you write

[31] if (OnLoadData==null) {
[32]     OnLoadData(this,argument)
[33] }

Unfortunately,  there are two little problems with that.  First,  none of us program in a vacuum,   so many of us will end up having to maintain or use objects where somebody forgot to include a null check.   Secondly,  the example between lines [31] and [33] isn’t thread safe.  It’s possible that a method can be removed from OnLoadData between the time of the null check and the call!

It turns out that extension methods can be added to delegates,  so Chris created a really nice extension method called Fire() that encapsulates the error check code between 31-33.   Now you can just write the code you wanted to write:

[34] OnLoadData.Fire(this,argument);

and be confident that knowledge about threads and quirks of the type system is embedded in an extension method.

You must use this to access an extension method inside a subclass

Suppose you’re building a Silverlight application and you’d like your team to have an important method that incorporates something tricky on their fingertips.  For instance,  suppose you’re implementing error handling in an event handler that’s responding to a user-initiated event or an async callback.   You can always write

[35] if(... something wrong...) {
[36]    ... several lines of code to display dialog box ...
[37]    return;
[38] }

But this is something that (i) programmers don’t want to do to begin with,  (ii) that programmers will have to do tens or hundreds of times,  and (iii) isn’t going to be in the main line of testing.  It’s a quality problem waiting to happen.  It’s imperative,  therefore,  to reduce the amount of code to do the right thing as much as possible…  To make it easier to do the right thing than to do the wrong thing.   It’s tempting to define an extension method like:

[39] public static void ErrorDialog(this UserControl c, string message) {
[40]    throw new ErrorMessageException(message);
[41] }

and catch the ErrorMessageException in the global error handler.  (The “method that doesn’t return” is effective,  because it avoids the need to repeat the return,  which occassionaly seems to vanish when people write repetitive error handling code.)  You’d think that this simplifies the code inside the UserControls you write to:

[42] if (... something wrong...)  {
[43]    ErrorDialog(...);
[44] }

But it turns out that line [43] doesn’t actually work,  and you need to write

[45] if (... something wrong...) {
[46]    this.ErrorDialog(...);
[47] }

in which case you might as well use an ordinary static method on a helper class.

What’s wrong with extension methods?

I’ve seen two arguments against extension methods:  (i) extension methods could make code hard to understand (and hence maintain) and (ii) extension methods are vulnerable to namespace conflicts.  I think (i) is a specious argument,  but (ii) is serious.

I think (i) splits into two directions.  First there’s the practical problem that a programmer is going to see some code like

[48] String s="somebody@example.com";
[49] if (s.IsValidEmailAddress()) {
[50]     ... do something ...
[51] }

and wonder where the heck IsValidEmailAddress() comes from,  where it’s documented,  and so forth.  Practically,  Visual Studio understands extension methods well,  so a user that clicks on “Go To Definition” is going to get a quick answer.

Going further,  however,  one can imagine that extension methods could transform C# unrecognizably:  I think of a friend of mine who,  in the 1980’s,  liked FORTRAN better than C,  and abused preprocessor macros so he could write C code that looked like FORTRAN.   This is connected with a fear of lambda expressions,  and other features that derive from functional programming.  For instance,  that beginning programmers just won’t get it.

We’ll see how it all works out,  but I think that new features in C# are going to help the language evolve in a way more like jquery and prototype have transformed javascript.  Microsoft is bringing concepts that have been locked in the ivory tower for decades into the mainstream:  all programming languages are going to benefit in the long term.

Extension methods,  precedence and namespaces

Here’s the killer.

I can make extension methods available by just adding a namespace to my .cs file with a using directive.  The compiler scans the namespace for extension methods in static classes,  and makes them available.  Pretty easy,  right?  Well,  what happens if two extension methods with the same name get declared in two namespaces which get included in the same file?  What if we define an extension method on class A,  but there’s a conventional method with the same name on class B?  What if file One.cs uses namesspace C,  and Two.cs uses namespace D,   so that ThisExtensionMethod means something different in One.cs and Two.cs?

There are real problems in how extension methods interact with namespaces.  These problems aren’t as fatal as namespace conflicts were in C (and C++ before namespaces),  but they are for real.

One answer is to avoid the use of extension methods entirely,  but that causes the loss of the benefits.  Anyone who uses extension methods should take a close look at the C# version 3.0 specification and think about how precedence rules effect their work:

(i) Instance methods take precedence over extension methods.  The definition of an instance method makes extension methods with the same name inaccessable.  This happens at the level of methods,  not method groups,  so two methods with the same name but different signatures can be handled by an extension method and instance method respectively.
(ii) Once the compiler tries extension methods,  processing works backwards from the closest enclosing namespace declaration outward,  trying extension methods defined in using groups.
(iii) The compiler throws an error when there are two or more extension methods that are candidates for a spot.

Matt Manela demonstrates an interesting example on the MSDN forums.  With three examples,  he demonstrates that the existence of an instance method (that overrides both extension methods) will suppress the generation of an error message about a conflict between extension methods.  This indicates that potentially conflicting extension methods in two namespaces will only cause an error if an attempt is made to use them.

Mitigating Namespace Conflicts

Overall,  conflicts between extension methods in different namespaces will not result in catastrophic consequences:

1. The compiler raises an error if there is any ambiguity as to which extension method to apply at a particular invocation — code won’t silently change behavior upon adding a new namespace in a using directive.
2. The compiler does not throw an error if potentially conflicting extension methods are declared in two different namespaces including in distinct using directives if those extension methods are not used — therefore,  conflicting extension methods won’t automatically prevent you from using any namespaces you choose.
3. If there is a conflict,  either between two extension methods or an extension method and an instance methods,  you can always call a specific extension method like an ordinary static example.  For instance,  in the case above:

ObjectExtension.IsNull(someObject);

You won’t end up in a situation where an extension method becomes unavailable because of a conflict — you’ll just be forced to use an uglier syntax.  I do see two real risks:

1. You can end up using an extension method that you don’t expect if you’re not keeping track of which using directives are in your file,  and
2. An instance method can silently shadow an extension method.  A change in the definition of a method could cause the behavior of a (former) extension method cal to change in a suprising way.  On the other hand,  this could be a useful behavior if you’d like a subclass to override a behavior defined in an extension method.

A common bit of advice that I’ve seen circulating is that extension methods should be defined in separate namespaces,  so that it would be possible to include or not include extension methods associated with a namespace to avoid conflicts.  I think this is based on superstition,  for,  as we’ve seen,  conflicting extension methods do not preclude the use of two namespaces;  this advice is certainly not followed in the System.Linq namespace,  which defines a number of valuable extension methods in the System.Linq.Enumerable static class.

Conclusion

We’re still learning how to use extension methods effectively.  Although extension methods have great promise,  they’re difference from ordinary instance methods in a number of ways.  Some of these,  like the difference in null handling,  are minor,  and could potentially be put to advantage.  Others,  such as the interaction with namespaces in large projects,   are more challenging.  It’s time to start building on our experiences to develop effective patterns for using extension methods.

The problem

Silverlight 2 Beta 2 has changed the concurrency model for asynchronous communications.  In Silverlight 2 Beta 1,  asynchronous requests always returned on the UI Thread.  This was convenient,  since updates to the user interface can only be done via the UI thread.  As of Silverlight 2 Beta 2,  asynchronous callbacks are fired in worker threads that come from a thread pool:  although this potentially allows for better performance via concurrency,  it increases the risk for race conditions between callbacks –  more importantly,  some mechanism is necessary to make code that updates the user interface run in the UI thread.

A solution

It’s straightforward to execute a function in the UI thread by using the Dispatcher property of any ScriptObject The tricky part is that ScriptObjects are part of the user interface,  so you can only access the Dispatcher property from the UI thread.  At first this seems like a chicken-and-egg situation:  you need a Dispatcher to get to the UI thread,  but you need to be in the UI thread to get a Dispatcher.

This dilemma can be resolved by accessing a Dispatcher in your App.xaml.cs file and stashing it away in a static variable on application startup:

private void Application_Startup(object sender, StartupEventArgs e) {
    ...
    UIThread.Dispatcher = RootVisual.Dispatcher;
}

UIThread is a simple static class:

public static class UIThread {
    public static Dispatcher Dispatcher {get; set;}
    public static void Run(Action a) {
        Dispatcher.BeginInvoke(a);
    }
}

At some point in the future,  you can transfer execution to the UIThread by scheduling a function to run in it.

public class ProcessHttpResponse(...) {
   ...
   UIThread.Run(UpdateUserInterface);
}

The parameter of Run is an Action delegate,  which is a function that returns void and takes no parameters.  That’s fine and good,  but what if you want to pass some parameters to the function that updates the UI.  The usual three choices for passing state in asynchronous applications apply,  but it’s particularly easy and fun to use a closure here:

public class ProcessHttpResponse(...) {
    ...
    var elementToUpdate=...;
    var updateWithValue=...;

    UIThread.Run(delegate() {
        UpdateUserInterface(elementToUpdate,updateWithValue)
    });
}

When to return?

If your application is complex,  and you have nested asynchronous calls,  you’re left with an interesting question:  where is the best place to switch execution to the UI thread?  You can switch to the UI Thread as soon as you get back from an HttpRequest or a WCF call and you must switch to the UI Thread before you access any methods or properties of the user interface.  What’s best?

It is simple and safe to switch to the UI Thread immediately after requests return from the server.  If you’re consistent in doing this,  you’ll never have trouble accessing the UI thread,  and you’ll never have trouble with race conditions between returning communication responses.  On the other hand,  you’ll lose the benefit of processing responses concurrently,  which can improve speed and responsiveness on today’s multi-core computers.

It’s straightforward to exploit concurrency when a requests can be processed independently.  For instance,  imagine a VRML viewer written in Silverlight.  Displaying a VRML would require the parsing of a file,  the construction of the scene graph and the initialization of a 3-d engine,  which may require building data structures such as a BSP Tree.  Doing all of this work in the UI Thread would make the application lock up while a model is loading — it would be straightforward,  instead,  to construct all of the data structures required by the 3-d engine,  and attach the fully initialized 3-d engine to the user interface as the last step.  Since the data structures would be independent of the rest of the application,  thread safety and liveness is a nonissue.

Matters can get more complicated,  however,  if the processing of a request requires access to application-wide data;  response handlers running in multiple threads will probably corrupt shared data structures unless careful attention is paid to thread safety.  One simple approach is to always access shared data from the UI Thread,  and to transfer control to the UI Thread with UIThread.Run before accessing shared variables.

Conclusion

Silverlight 2 Beta 2 introduces a major change in the concurrency model for asynchronous communication requests.  Unlike SL2B1,  where asynchronous requests executed on the user interface thread,  SL2B2 launches asynchronous callbacks on multiple threads.  Although this model offers better performance and responsiveness,  it requires Silverlight programmers to explicitly transfer execution to the UI thread before accessing UI objects:  most SL2B1 applications will need to be reworked.

This article introduces a simple static class,  UIThread,  which makes it easy to schedule execution in the UI Thread.

Introduction

This is a story of two types: GenericType and SpecificType, where GenericType is a superclass of SpecificType. There are two types of explicit cast in C#:

The Prefix cast:

[01] GenericType g=...;
[02] SpecificType t=(SpecificType) g;

The as cast:

[03] GenericType g=...;
[04] SpecificType t=g as SpecificType;

Most programmers have a habit of using one or the other — this isn’t usually a conscious decision, but more of a function of which form a programmer saw first. I, for instance, programmed in Java before I learned C#, so I was already in the prefix cast habit. People with a Visual Basic background often do the opposite. There are real differences between the two casting operators which can make a difference in the reliability and performance of your application.

Prefix casting: Reliable Casting

The major difference between prefix- and as-casting is what happens when the cast fails. Imagine, for instance, that g is really an instance of AnotherSpecificType, which is not a subclass of SpecificType. In this case, the prefix-cast throws an exception at line [2] — however, the as-cast returns null when the cast fails, letting the execution of the program barrel on.

It’s easier to implement correct error handling in programs that use prefix-casting, and programs that use prefix-casting are easier to debug. Prefix-casting causes an exception to be thrown at the moment of the cast, where the problem is obvious. As-casting, on the other hand, can cause an exception to be thrown at the moment where the SpecificType t is referenced. The used of the SpecificType can be far away in the program: it can be in another method, or another class — it could even happen hours after the cast is performed. Be it in development or production, bugs caused by corrupted data structures can be maddeningly difficult to find and correct.

As-casting: Fast Casting

If it’s harder to write correct programs with as-casting, why would anybody use it? For one thing, as-casting is faster than prefix casting by quite a lot. Benchmarks show that as-casting is about five times faster than prefix casting. That said, the performance of casting is only going to matter in the innermost of loops of most programs. The fastest kind of casting is no casting at all: it’s best to use generics to eliminate the need for casting when possible and to move casting outside loops. (Generics also improve the reliability of your code because they help C#’s static type checking catch mistakes.)

There are some cases where as-casting could be convenient, for instance, when you expect the cast to fail sometimes. Often I like to ‘tag’ classes with interfaces that specify certain behaviors. For example,

[05] public Interface IBoppable {
[06]     void Bop();
[07] }

Now i might want to loop through a bunch of Thing objects, and bop all the ones that implement IBoppable: it’s reasonable to use as-casting here:

[08] List<Thing> list=...
[09] foreach(Thing thing in list) {
[10]    List boppable=thing as IBoppable;
[11]    if (boppable !=null) {
[12]        boppable.Bop()
[13]    }
[14] }

It’s OK to use as-casting if you’re going to check to see if the value is null immediately after the cast. The above code is correct, but has the bad smell that the boppable variable continues to exist in the block after the cast… It’s still there for a later maintainer to use erroneously. In cases like this, code can be made clearer with the is operator:

[15] List<Thing> list=...
[16] foreach(Thing thing in list) {
[17]    if(thing is IBoppable) {
[18]        ((IBoppable) boppable).Bop()
[19]    }
[20] }

(Speed freaks can use as-cast on line 18, as we know it’s not going to fail.)

The pattern of testing for null after an as-cast has a few problems. (i) It doesn’t distinguish between the case of the original object being null from the case of the original object being the wrong type and (ii) correct error handling often requires more contorted logic than using an exception — and once you added test logic, you’ve lost the speed advantage of as-casting.

Conclusion

C# offers two casting operators: the prefix-cast and the as-cast. Although the two operators compile to different op-codes in the CLR, the practical difference between them is in how they handle failed casts. Prefix-cast throws an exception on cast failure, while as-cast returns null. It’s easier to implement correct error handling when you use prefix cast, because it doesn’t require manual checks for null values that can cause problems in distant parts of your program. Prefix-cast should be the default cast operator on your fingertips, that you use for everyday situations — reserve as-cast for special cases where performance matters. (For best performance, however, eliminate the cast entirely.)

When you’re writing applications that use asynchronous callbacks (i.e. Silverlight, AJAX, or GWT) you’ll eventually run into the problem of keeping track of the context that a request is being done in. This isn’t a problem in synchronous programming, because local variables continue to exist after one function calls another function synchronously:

int AddToCount(int amount,string countId)  {
   int countValue=GetCount(countId);
   return countValue+amount;
}

This doesn’t work if the GetCount function is asynchronous, where we need to write something like

int AddToCountBegin(int amount,string countId,CountCallback outerCallback) {
     GetCountBegin(countId,AddToCountCallback);
}

void AddToCountCallback(int countValue) {
    ... some code to get the values of amount and outerCallback ...
    outerCallback(countValue+amount);
}

Several things change in this example: (i) the AddToCount function gets broken up into two functions: one that does the work before the GetCount invocation, and one that does the work after GetCount completes. (ii) We can’t return a meaningful value from AddToCountCallback, so it needs to ‘return’ a value via a specified callback function. (iii) Finally, the values of outerCallback and amount aren’t automatically shared between the functions, so we need to make sure that they are carried over somehow.
There are three ways of passing context from a function that calls and asynchronous function to the callback function:

1. As an argument to the callback function
2. As an instance variable of the class of which the callback function is a class
3. Via a closure

Let’s talk about these alternatives:

1. Argument to the Callback Function

In this case, a context object is passed to the asynchronous function, which passes the context object to the callback. The advantage here is that there aren’t any constraints on how the callback function is implemented, other than by accepting the context object as a callback. In particular, the callback function can be static. A major disadvantage is that the asynchronous function has to support this: it has to accept a state object which it later passes to the callback function.

The implementation of HttpWebRequest.BeginGetResponse(AsyncCallback a,Object state) in the Silverlight libraries is a nice example. If you wish to pass a context object to the AsyncCallback, you can pass it in the second parameter, state. Your callback function will implement the AsyncCallback delegate, and will get something that implements IAsyncResult as a parameter. The state that you passed into BeginGetResponse will come back in the IAsyncResult.AsyncState property. For example:

class MyHttpContext {
	public HttpWebRequest Request;
        public SomeObject FirstContextParameter;
        public AnotherObject AnotherContextParameter;
}

protected void myHttpCallback(IAsyncResult abstractResult) {
	MyHttpContext context = (MyHttpContext) abstractResult.AsyncState;
	HttpWebResponse Response=(HttpWebResponse) context.Request.EndGetResponse(abstractResult);
}

public doHttpRequest(...) {
	...
        MyHttpContext context=new MyHttpContext();
	context.Request=Request;
	context.FirstContextParameter = ... some value ...;
	context.AnotherContextParameter = .. another value ...;
	Request.BeginGetResponse();
	Request.Callback(myHttpCallback,context);
}

Note that, in this API, the Request object needs to be available in myHttpCallback because myHttpCallbacks get the response by calling the HttpWebResponse.EndGetResponse() method. We could simply pass the Request object in the state parameter, but we’re passing an object we defined, myHttpCallback, because we’d like to carry additional state into myHttpCallback.

Note that the corresponding method for doing XMLHttpRequests in GWT, the use of a RequestBuilder object doesn’t allow using method (1) to pass context information — there is no state parameter. in GWT you need to use method (2) or (3) to pass context at the RequestBuilder or GWT RPC level. You’re free, of course, to use method (1) when you’re chaining asynchronous callbacks: however, method (2) is more natural in Java where, instead of a delegate, you need to pass an object reference to designate a callback function.

2. Instance Variable Of The Callback Function’s Class

Functions (or Methods) are always attached to a class in C# and Java: thus, the state of a callback function can be kept in either static or instance variables of the associated class. I don’t advise using static variables for this, because it’s possible for more than one asynchronous request to be flight at a time: if two request store state in the same variables, you’ll introduce race conditions that will cause a world of pain. (see how race conditions arise in asynchronous communications.)

Method 2 is particularly effective when both the calling and the callback functions are methods of the same class. Using objects whose lifecycle is linked to a single asynchronous request is an effective way to avoid conflicts between requests (see the asynchronous command pattern and asynchronous functions.)

Here’s an example, lifted from the asynchronous functions article:

    public class HttpGet : IAsyncFunction<String>
    {
        private Uri Path;
        private CallbackFunction<String> OuterCallback;
        private HttpWebRequest Request;

        public HttpGet(Uri path)
        {
            Path = path;
        }

        public void Execute(CallbackFunction<String> outerCallback)
        {
            OuterCallback = outerCallback;
            try
            {
                Request = (HttpWebRequest)WebRequest.Create(Path);
                Request.Method = "GET";
                Request.BeginGetRequestStream(InnerCallback,null);
            }
            catch (Exception ex)
            {
                OuterCallback(CallbackReturnValue<String>.CreateError(ex));
            }
        }

        public void InnerCallback(IAsyncResult result)
        {
            try
            {
                HttpWebResponse response = (HttpWebResponse) Request.EndGetResponse(result);
                TextReader reader = new StreamReader(response.GetResponseStream());
                OuterCallback(CallbackReturnValue<String>.CreateOk(reader.ReadToEnd()));
            } catch(Exception ex) {
                OuterCallback(CallbackReturnValue<String>.CreateError(ex));
            }
        }
    }

Note that two pieces of context are being passed into the callback function: an HttpWebRequest object named Request (necessary to get the response) and a CallbackFunction<String> delegate named OuterCallback that receives the return value of the asynchronous function.

Unlike Method 1, Method 2 makes it possible to keep an unlimited number of context variables that are unique to a particular case in a manner that is both typesafe and oblivious to the function being called — you don’t need to cast an Object to something more specific, and you don’t need to create a new class to hold multiple variables that you’d like to pass into the callback function.

Method 2 comes into it’s own when it’s used together with polymorphism, inheritance and initialization patterns such as the factory pattern: if the work done by the requesting and callback methods can be divided into smaller methods, a hierarchy of asynchronous functions or commands can reuse code efficiently.

3. Closures

In both C# and Java, it’s possible for a method defined inside a method to have access to variables in the enclosing method. In C# this is a matter of creating an anonymous delegate, while in Java it’s necessary to create an anonymous class.

Using closures results in the shortest code, if not the most understandable code. In some cases, execution proceeds in a straight downward line through the code — much like a synchronous version of the code. However, people sometimes get confused the indentation, and, more seriously, parameters after the closure definition and code that runs immediately after the request is fired end up in an awkward place (after the definition of the callback function.)

    public class HttpGet : IAsyncFunction<String>
    {
        private Uri Path;

        public HttpGet(Uri path)
        {
            Path = path;
        }

        public void Execute(CallbackFunction<String> outerCallback)
        {
            OuterCallback = outerCallback;
            try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Path);
                Request.Method = "GET";
                Request.BeginGetRequestStream(delegate(IAsyncResult result) {
	            try {
                        response = request.EndGetResponse(result);
                        TextReader reader = new StreamReader(response.GetResponseStream());
                        outerCallback(CallbackReturnValue<String>.CreateOk(reader.ReadToEnd()));
                    } catch(Exception ex) {
                        outerCallback(CallbackReturnValue<String>.CreateError(ex));
                    }

            },null); // <--- note parameter value after delegate definition

            }
            catch (Exception ex)
            {
                outerCallback(CallbackReturnValue<String>.CreateError(ex));
            }
        }
    }

The details are different in C# and Java: anonymous classes in Java can access local, static and instance variables from the enclosing context that are declared final — this makes it impossible for variables to be stomped on while an asynchronous request is in flight. C# closures, on the other hand, can access only local variables: most of the time this prevents asynchronous requests from interfering with one another, unless a single method fires multiple asynchronous requests, in which case counter-intuitive things can happen.

Conclusion

In addition to receiving return value(s), callback functions need to know something about the context they run in: to write reliable applications, you need to be conscious of where this information is; better yet, a strategy for where you’re going to put it. Closures, created with anonymous delegates (C#) or classes (Java) produce the shortest code, but not necessarily the clearest. Passing context in an argument to the callback function requires the cooperation of the called function, but it makes few demands on the calling and callback functions: the calling and callback functions can both be static. When a single object contains both calling and callback functions, context can be shared in a straightforward and typesafe manner; and when the calling and callback functions can be broken into smaller functions, opportunities for efficient code reuse abound.

For years people have talked about an “impedance mismatch” between relational databases and object-oriented languages. Big answers have been proposed to this problem, such as object-relational mapping, but we’ve yet to see a big answer that’s entirely satisfactory — this article is one of a series that tries to break the problem into a set of smaller problems that have simple answers.

Many modern languages, such as Java, Perl, PHP and C#, are derived from C. In C, string literals are written in double quotes, and a set of double quotes can’t span a line break. One advantage of this is that the compiler can give clearer error messages when you forget the close a string. Some C-like languages, such as Java and C# are like C in that double quotes can’t span multiple lines. Other languages, such as Perl and PHP, allow it. C# adds a new multiline quote operator, @”" which allows quotes to span multiple lines and removes the special treatment that C-like languages give to the backslash character (\).

The at-quote is particularly useful on the Windows platform because the backslash used as a path separator interacts badly with the behavior of quotes in C, C++ and many other languages. You need to write

String path="C:\\Program Files\\My Program\\Some File.txt";

With at-quote, you can write

String path=@"C:\Program Files\My Program\Some File.txt";

which makes the choice of path separator in MS-DOS seem like less of a bad decision.

Now, one really great thing about SQL is that the database can optimize complicated queries that have many joins and subselects — however, it’s not unusual to see people write something like

command.CommandText = "SELECT firstName,lastName,(SELECT COUNT(*) FROM comments WHERE postedBy
=userId AND flag_spam='n') AS commentCount,userName,city,state,gender,birthdate FROM user,user
Location,userDemographics WHERE user.userId=userLocation.userId AND user.userId=userDemographi
cs.user_id AND (SELECT COUNT(*) FROM friendsReferred WHERE referringUser=userId)>10 ORDER BY c
ommentCount HAVING commentCount>3";

(line-wrapped C# example) Complex queries like this can be excellent, because they can run thousands of times faster than loops written in procedural or OO languages which can fire off hundreds of queries, but a query written like the one above is a bit unfair to the person who has to maintain it.

With multiline quotes, you can continue the indentation of your code into your SQL:

command.CommandText = @"
   SELECT
       firstName
      ,lastName
      ,(SELECT COUNT(*) FROM comments
         WHERE postedBy=userId AND flag_spam='n') AS commentCount
      ,userName
      ,city
      ,state
      ,gender
      ,birthdate
   FROM user,userLocation,userDemographics
   WHERE
      user.userId=userLocation.userId
      AND user.userId=userDemographics.user_id
      AND (SELECT COUNT(*) FROM friendsReferred
         WHERE referringUser=userId)>10
   ORDER BY commentCount
   HAVING commentCount>3
";

Although this might not be quite quite as cool as LINQ, it works with Mysql, Microsoft Access or any database you need to connect to — and it works in many other languages, such as PHP (in which you could use ordinary double quotes.) In languages like Java that don’t support multiline quotes, you can always write

String query =
   " SELECT"
  +"    firstName"
  +"   ,lastName"
  +"   ,(SELECT COUNT(*) FROM comments"
  +"      WHERE postedBy=userId AND flag_spam='n') AS commentCount"
  +"   ,userName"
  +"   ,city"
  +"   ,state"
  +"   ,gender"
  +"   ,birthdate"
  +" FROM user,userLocation,userDemographics"
  +"    WHERE"
  +"       user.userId=userLocation.userId"
  +"       AND user.userId=userDemographics.user_id"
  +"       AND (SELECT COUNT(*) FROM friendsReferred"
  +"          WHERE referringUser=userId)>10"
  +" ORDER BY commentCount"
  +" HAVING commentCount>3";

but that’s a bit more cumbersome and error prone.

CORRECTION:  The threading model in Silverlight has changed as of Silverlight 2 Beta 2.  It is now possible to initiate asynchronous communication from any thread,  however,  asynchronous callbacks now run in “new” threads that come from a thread pool.  The issues in this article still apply,  with two additions:  (1) the possibility of race conditions and deadlocks between asynchronous callback threads and (2) all updates to user interface components must be done from the user interface thread.  (Fortunately,  it’s easy to get back to the UI thread.)  Subscribe to our RSS Feed to keep informed of breaking developments in Silverlight development.

There’s a lot of confusion about how asynchronous communication works in RIA’s such as Silverlight, GWT and Javascript. When I start talking about the problems of concurrency control, many people tell me that there aren’t any concurrency problems since everything runs in a single thread. [1]

It’s important to understand the basics of what is going on when you’re writing asynchronous code, so I’ve put together a simple example to show how execution works in RIA’s and how race conditions are possible. This example applies to Javascript, Silverlight, GWT and Flex, as well as a number of other environments based on Javascript. This example doesn’t represent best practices, but rather what can happen when you’re not using a proactive strategy that eliminates concurrency problems:

In the diagram above, execution starts when the user pushes a button (a). This starts the user interface thread by invoking an onClick handler. The user interface thread starts two XmlHttpRequests, (b) and (c). The event handler eventually returns, so execution stops in the user interface thread.

In the meantime, the browser still has two XmlHttpRequests running. Callbacks from http requests, timers and user interfaces go into a queue — they get executed right away if the user interface thread is doing nothing, but get delayed if the user interface thread is active.

Http request (b) completes first, causing the http callback for request (b) to start. Had something been a little different with the web browser, web server or network, request (c) could have returned first, causing the callback for request (c) to start. If the result of the program depends on the order that the callbacks for (b) and (c) run, we have a race condition. The callback for http request (b) starts a new http request (d), which runs for a long time.

In the meantime, the user is moving the mouse and triggers a mouseover event while the request (b) callback is running. Right after the request (b) callback completes, the web browser starts the UI thread, which causes a mouseover event handler (e) to run. Note that the user can trigger user interface events while XmlHttpRequests are running, causing event handlers to run in an unpredictable order: if this causes your program to malfunction, your program has a bug.

While the event handler (e) is running, request (c) completes: like the mouseover event, this event is queued and runs once event handler (e) completes. Before (e) completes, it starts a new http request (f). The browser looks into the event queue when (e) completes, and starts the callback for (c). Http request (f) completes while callback (c) is running, gets queued, and runs after (c) is running.

At the end of this example, the callback for (f) completes, causing the UI thread to stop. The http request (c) is still in flight — it completes in the future, somewhere off the end of the page.

This example did not include any timers, or any mechanism of deferred execution such as DeferredCommand in GWT or Dispatcher.Invoke() in Silverlight. This is but another mechanism to add callback references to the event queue.

As you can see, there’s a lot of room for mischief: http requests can return in an arbitrary order and users can initiate events at arbitrary times. The order that things happen in can depend on the browser, it’s settings, on the behavior of the server, and everything in between. Some users might use the application in a way that avoids certain problems (they’ll think it’s wonderful) and others might consistently or occasionally trigger an event that causes catastrophe. These kind of bugs can be highly difficult to reproduce and repair.

Asynchronous RIAs have problems with race conditions that are similar to threaded applications, but not exactly the same. Today’s languages and platforms have excellent and well documented mechanisms for dealing with threads, but today’s RIAs do not have mature mechanisms for dealing with concurrency. Over time we’ll see libraries and frameworks that help, but asynchronous safety isn’t something that can be applied like deodorant: it involves non local interactions between distant parts of the program. The simplest applications can dodge the bullet, but applications beyond a certain level of complexity require an understanding of asynchronous execution and the consistent use of patterns that avoid trouble.

[1] Although it is possible to create new threads in Silverlight, all communication and user interface access must be done from the user interface thread — many Silverlight applications are single-threaded, and adding multiple threads complicates the issue.

Asynchronous Commands are a useful way to organize asynchronous activities, but they don’t have any way to pass values or control back to a caller. This post contains a simple Asynchronous Function library that lets you do that. In C# you call an Asynchronous Function like:

 void CallingMethod(...) {
    ... do some things ...
    IAsyncFunction<String> httpGet=new HttpGet(... parameters...);
    anAsynchronousFunction.Execute(CallbackMethod);
}

void CallbackMethod(CallbackReturnValue<String> crv) {
    if (crv.Error!=null) { ... handle Error,  which is an Exception ...}
    String returnValue=crv.Value;
    ... do something with the return value ...
}

We’re using generics so that return values can be passed back in a type safe manner. The type of the return value of the asynchronous function is specified in the type parameter of IAsyncFunction and CallbackReturnValue.

Asynchronous functions catch exceptions and pass them back in  in the CallbackReturnValue.  This makes it possible to propagate exceptions back to the caller,  as in synchronous functions.  The code to do this must has to be manually replicated in each asynchronous function,  however,  the code can be put into a wrapper delegate.

You could do the same thing in Java, but the CallbackMethod would need to be a class that implements an interface rather than a delegate.

Continue Reading »

When you’re writing RIA applications in an environment like Silverlight or GWT, you’re restricted to doing asynchronous http calls to the server — this leaves you with a number of tricky choices, such as, where to put your callback functions. To be specific, imagine we’ve created a panel in the user interface where a user enters information, then clicks on a form to submit it. The first place you might think of putting the callback is in the class for the panel, something like

public class MyPanel:StackPanel {
	... other functions ...

        void SubmitButton_Click(Object sender,EventArgs e) {
           ... collect data from forms ...
           ServerWrapper.DoSubmission(formData,SubmissionCallback);
        }

        void SubmissionCallback(SubmissionResult result) {
           ... update user interface ...

        }

}

(Although code samples are in C#, the language I’m using now, I developed this pattern when working on a Java project.) This is a straightforward pattern for the simplest applications, but it runs out of steam when your application becomes more complex. It can become confusing to keep track of your callback functions when your object does more than one kind of asynchronous call: for instance, if it has multiple buttons. If the same action can be done on the server from more than one place in the UI, it’s not clear where the callback belongs.

One answer to the problem is to use the Command Pattern, to organize asynchronous activities into their own classes that contain both the code that initiates an asynchronous request and the callback that runs when the request completes. Continue Reading »