Background

I recently started evaluating MVVM libraries for my client’s project. Though I often share parts of my own unpublished library with a client I had inherited this project and it already had it’s share of commonly-used MVVM types. What it lacked, and I added, was a quickie "message bus" implementation. Not the distributed, enterprise-class sort of message bus, but an intra-app messaging service that decouples senders from receivers. It’s the message bus that I really wanted to replace with a more robust, tested version that must surely already exist in one of the established MVVM libraries.

As luck would have it the first library I downloaded touted a messaging service but, as is too often the case, you get what you pay for. After perusing the code for a bit I could see that the messaging functionality wasn’t thread-safe, a feature I require. My app is a multi-threaded WPF app with both managed and native threads and takes pains to keep work off the UI thread in order to achieve smooth and somewhat real-time rendering of data. Wrapping the library’s messaging code for synchronization would be easy, but it doesn’t feel like fine enough granularity of locking for good performance.

Weak Action is Weak

Another driver in looking for an established library is that my quickie message bus implementation holds references to subscribed listeners. Not really a problem unless a listener fails to unsubscribe, in this case the listener will never be garbage collected. I consider that an undesirable situation. So I took a look at the above library’s implementation of a weak action; I quickly saw that this code used not a weak reference but a strong reference; it suffered the very problem it purported to fix. FYI, I did verify that the problem is known to the library’s author.

Now, at this time I haven’t finished evaluating libraries, and this was a bit of a rough start. Maybe I’ll just set aside some time to come up with a solution myself. Like… now.

An Elegant Solution (maybe…)

The problem I’d like to solve is that Action holds a reference to the target object of non-static delegates, preventing that target from being GC’d. The most elegant solution would allow me to simply replace my use of Action and Action<T> with an implementation that holds only a weak reference to the target. And here it is:

   1: static Action CreateWeakAction(Action action)

   2: {

   3:     if (action == null)

   4:         throw new ArgumentNullException("action");

   5:  

   6:     // If it's a static delegate there's no need to create a weak reference.

   7:     if (action.Target == null)

   8:         return action;

   9:  

  10:     Type type = action.GetType();

  11:     WeakReference targetRef = new WeakReference(action.Target);

  12:     MethodInfo method = action.Method;

  13:  

  14:     return () =>

  15:         {

  16:             // Don't reference 'action' in this lambda expression.

  17:             object target = targetRef.Target;

  18:             if (target == null)

  19:             {

  20:                 // Target was GC'd; don't do anything. (Ick.)

  21:             }

  22:             else

  23:             {

  24:                 Delegate.CreateDelegate(type, target, method).DynamicInvoke();

  25:             }

  26:         };

  27: }

Using CreateWeakAction is pretty easy:

   1: Action action = CreateWeakAction(() => receiver.DoSomething(42));

   2: // Time passes...

   3: action();

Now the target (‘receiver’ in the above example) is free to be collected, but I have a new Action delegate returned from CreateWeakAction that will not be collected unless I release my reference to it. More than that, I have no way of telling whether the delegate I’m holding no longer has a live reference to the target. I could conceivably change my Action to a Func<> that returns an indicator of whether the target was GC’d, but that would require me to actually call the delegate to determine its state. And I would lose the elegant drop-in replacement I was hoping for.

A More Measured Solution

Setting aside my personal desire for elegance solutions, the actual requirements I’m looking to satisfy in my message bus are twofold: 1) don’t hold a reference to the target listener and 2) don’t hold unnecessary objects in the heap. A weak action implementation that satisfies these needs might look something like this:

   1: class WeakAction

   2: {

   3:     public WeakAction(Action action);

   4:     public bool IsTargetAlive { get; }

   5:     public WeakActionInvocationResult Invoke();

   6: }

This would allow me to create an instance of my weak action, invoke it’s behavior when needed, and check whether the target is still alive so that I can cull stale instances from my message bus implementation. And, thanks to an uninterrupted morning at my local coffee joint I have what I wanted. You may note that I’ve made an abstraction for invoking instance delegates; this makes that code more easily reusable for generic implementations (e.g., WeakAction<T>).

   1: internal class WeakAction

   2: {

   3:     private readonly InvocationAgent _instanceAction;

   4:     private readonly Action _staticAction;

   5:  

   6:     public WeakAction(Action action)

   7:     {

   8:         if (action == null)

   9:             throw new ArgumentNullException("action");

  10:  

  11:         if (action.Target != null)

  12:         {

  13:             _instanceAction = new InvocationAgent(action);

  14:         }

  15:         else

  16:         {

  17:             _staticAction = action;

  18:         }

  19:  

  20:         Debug.Assert(_instanceAction == null || _staticAction == null);

  21:         Debug.Assert(!(_instanceAction == null && _staticAction == null));

  22:     }

  23:  

  24:     public bool IsTargetAlive { get { return IsStatic || _instanceAction.IsTargetAlive; } }

  25:  

  26:     private bool IsStatic { get { return _staticAction != null; } }

  27:  

  28:     public WeakActionInvocationResult Invoke()

  29:     {

  30:         if (_instanceAction != null)

  31:             return _instanceAction.Invoke();

  32:  

  33:         _staticAction();

  34:         return WeakActionInvocationResult.Invoked;

  35:     }

  36: }

Note that I’ve made an optimization for static delegates—there’s no need to incur the cost of dynamic invocation in that case because a static delegate has no target, therefore it won’t prevent anything from being GC’d.

Now imagine we have a messaging implementation that needs to track listeners with Action delegates but would rather not keep a reference to listeners forever if they happen not to unsubscribe. We can now create an instance of WeakAction and use it to invoke the listener’s Action when we wish; we can also query the WeakAction on occasion to determine whether it still has a live listener.

Finally here are the missing parts to make it work, the interesting bits. What makes this work is pulling apart the delegate into its component parts, holding the target part with a  weak reference, and reassembling the parts into a delegate only when needed to invoke the original delegate. No reference to the original delegate is held.

   1: internal enum WeakActionInvocationResult

   2: {

   3:     Invoked,

   4:     Collected,

   5: }

   6:  

   7: internal class InvocationAgent

   8: {

   9:     private readonly Type _type;

  10:     private readonly WeakReference _targetRef;

  11:     private readonly MethodInfo _method;

  12:  

  13:     public InvocationAgent(Delegate action)

  14:     {

  15:         Debug.Assert(action.Target != null, "Expected a non-static delegate");

  16:  

  17:         _type = action.GetType();

  18:         _targetRef = new WeakReference(action.Target);

  19:         _method = action.Method;

  20:     }

  21:  

  22:     public bool IsTargetAlive { get { return _targetRef.IsAlive; } }

  23:  

  24:     public WeakActionInvocationResult Invoke(params object[] args)

  25:     {

  26:         object target = _targetRef.Target;

  27:         if (target == null)

  28:             return WeakActionInvocationResult.Collected;

  29:  

  30:         // Don't keep a reference to this delegate.

  31:         Delegate.CreateDelegate(_type, target, _method).DynamicInvoke(args);

  32:         return WeakActionInvocationResult.Invoked;

  33:     }

  34: }

I’ll leave it as an interesting challenge to the reader to implement WeakAction<T>. Here’s what it might look like:

   1: class WeakAction<T>

   2: {

   3:     public WeakAction(Action<T> action);

   4:     public bool IsTargetAlive { get; }

   5:     public WeakActionInvocationResult Invoke(T t);

   6: }

Grab a cup of coffee and have at it. Please note: I’ve tested only on the desktop CLR, not Silverlight or Phone, so caveat emptor.

    new MyObject().LongRunningMethod();

as well as this scenario: 

    MyObject o = new MyObject();    o.LongRunningMethod();

Here we’ll discuss how this could become a problem. Frankly, it’s pretty easy to cause the problem, but I think it generally involves some ugliness on the part of software design or implementation, largely involving clean-up of fields when the instance is finalized. 

Don’t Try This at Home

I don’t think it’s likely you’ll see a lot of code that does this, but here’s one way to run afoul of your object going way: export a reference to a field that the object cleans up when it’s finalized. You can export the reference, say, by making that field visible through a property. Exposing a field that you’re going to clean up in the finalizer would be an easy way to create a coupling between your object and other (arbitrary) client code that has no knowledge of your object’s life span. 

Call Stack Antics

Another way to invoke the potential problem is to lose your “this” reference on the call stack. The code below manages to lose the “this” reference by passing a field to the helper method rather than allowing the helper to access the field directly via its own “this” reference. When the helper tries to access fs.Length an ObjectDisposedException is thrown. 

Why does this throw? Well, the last live reference to the instance was lost when LongRunningMethod passed _input to Helper. Essentially we’ve again exported the field value from the instance and no longer hold a reference to the instance, allowing the GC to finalize it. Helper is left holding a reference to an object that has been finalized. 

Note: Again, you will not see this behavior in a debug build. When running code marked as “debug” the JITter extends the lifetime of the local object to the end of the method. So you will not see this effect if you’ve compiled with the /debug flag. 

    using System;    using System.IO;

    // Ugliness ensues.
    sealed class MyUglyObject
    {
        public MyUglyObject(string inputPath)
        {
            // Real production code would likely not delete the file when done...
            // ...but this is a sample app.
            _input = new FileStream(Path.GetTempFileName(), FileMode.Open, FileAccess.Read, FileShare.Read);
        }

        ~MyUglyObject()
        {
            if (_input != null)
            {
                _input.Close();
                _input = null;
            }
        }

        public void LongRunningMethod()
        {
            Helper(_input); // Our last reference to 'this' (implicit).
        }

        private void Helper(FileStream fs)
        {
            // A long-running method can easily experience a garbage collection
            // before returning. This one happens for force it to occur.
            GC.Collect();
            GC.WaitForPendingFinalizers();

            // Ka-boom!
            long inputSize = fs.Length;
            // ...
        }

        private FileStream _input;
    }

    class Program
    {
        static void Main(string[] args)
        {
            new MyUglyObject(@"....readme.txt").LongRunningMethod();
        }
    }

Can You See It?

As you can see above it takes a bit of effort to cause the code to blow up. If Helper had used _input.Length instead of taking a parameter the problem would not exist. 

But, what I find a bit creepy about the above code is that if Helper were a static method it would seem respectable: 

    private static void Helper(FileStream fs)
    {
        // A long-running method can easily experience a garbage collection
        // before returning. This one happens for force it to occur.
        GC.Collect();
        GC.WaitForPendingFinalizers();

        // Ka-boom!
        long inputSize = fs.Length;
        // ...
    }

At first glance it now looks like helper is a normal static helper function, as is likely seen in code bases across the world. It doesn’t need a “this” reference, it takes a reference to the object it uses, everything appears fine on the surface. Would you see the “this” reference being lost by the code calling Helper in a code review? I’m not so sure I would have until recently. 

There’s a regular idiom in C# in which we call a method on an object instance that we’ve created inline:

    new MyObject().LongRunningMethod();

There assumption may be an assumption that the lifetime of this instance of MyObject extends at least until LongRunningMethod returns, but this isn’t necessarily true. This same assumption is often made about local references to objects:

    MyObject o = new MyObject();    o.LongRunningMethod();

However, in both these cases the object instance may be collected before LongRunningMethod returns.

Does this really happen?

Yes, it can and does. The code below exercises this behavior. When you run it you will see the following output, indicating that the object was collected and finalized before LongRunningMethod returns:

    Using release build
    Inline
    Entering MyObject1.LongRunningMethod().
    Finalizing in ~MyObject1().
    Returning from MyObject1.LongRunningMethod().     Local reference
    Entering MyObject1.LongRunningMethod().
    Finalizing in ~MyObject1().
    Returning from MyObject1.LongRunningMethod().Note: You will not see this behavior in a debug build. When running code marked as “debug” the JITter extends the lifetime of the local object to the end of the method. So you will not see this effect if you’ve compiled with the /debug flag.

Note also that this article also assumes we’re using CLR 2.0. Future versions could obviously behavior differently.

Here’s the code. Just drop it into test.cs, run csc test.cs, and execute test.exe.

    using System;

    sealed class MyObject
    {
        ~MyObject()
        {
            Console.WriteLine("Finalizing in ~MyObject1().");
        }

        public void LongRunningMethod()
        {
            Console.WriteLine("Entering MyObject1.LongRunningMethod().");

            // A long-running method can easily experience a garbage collection
            // before returning. This one happens for force it to occur.
            GC.Collect();
            GC.WaitForPendingFinalizers();

            Console.WriteLine("Returning from MyObject1.LongRunningMethod().");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
    #if DEBUG
            string build = "debug";
    #else
            string build = "release";
    #endif
            Console.WriteLine("Using {0} build", build);

            // Try it both ways.

            Console.WriteLine("Inline");
            new MyObject().LongRunningMethod();

            Console.WriteLine();
            Console.WriteLine("Local reference");
            MyObject o = new MyObject();
            o.LongRunningMethod();
        }
    }

Is this a problem?

Generally, I’d say it’s not a problem. Once it has begun execution, LongRunningMethod doesn’t need the original object reference unless it’s making reference to that instance. In that case the GC won’t be able to collect the object.

I’ll discuss how to make it a problem in Part 2 of this article.

It doesn’t. It’s simply not type safe.

The Problem

Here are the types from the above-mentioned post; I’ve added the FavoriteColor property for later discussion:

For discussion’s sake, here’s the declaration of the method in question:

FetchContact‘s signature indicates that it can return a Contact via the contact parameter.

And now the code the poster wants to use:

It certainly appears that the calling code desires to get a Recipient from the call to FetchContact, but that’s not within the interface contract of FetchContact. FetchContact returns a Contact via its contact ref parameter.

Try, Try Again

Now, it’s true that the following fragment of code works; it adheres to the interface contract specified by FetchContact:

Well, of course that works, we’ve modified the code to pass a ref Contact as specified by the method signature. But why doesn’t the first?

Type Safety

Let’s try a different tack. To simplify, ignore for the moment that we’re looking at a ref parameter. Instead, consider it an out parameter. And we’ll also assume that there are more types derived from Contact, for example:

Now, consider again, what type FetchContact is returning here as an out parameter?

The correct answer is “We’re not sure.” FetchContact could return a Contact, a Recipient, a Sender, or any other type derived from Contact. The problem of the original code is that it assumes that FetchContact is returning a Recipient and tries to coerce the type. This would not be typesafe as we don’t know that the object returned is actually a Recipient.

Consider what happens when FetchContact is (legally) implemented like this:

The object passed back to the caller is obviously a Sender, not a Recipient. The only way for the caller to treat the returned value as a Recipient is to break type safety, and C# generally does not let one do this.

Hypothetically Speaking…

What if C# did let us ignore type safety? Let’s suppose that this code could compile and execute:

Now FetchContact has returned a Sender that we’ve coerced into a Recipient reference. Not good. It might execute right up until we use the FavoriteColor property. Think about it. FetchContact returns an instance of Sender, which we’re treating as a Recipient. But the Sender type doesn’t have a FavoriteColor property. What would happen then? It would surely crash or return invalid data, just as we would see in C++.

C# prefers type safety and doesn’t allow this to happen. That’s a good thing.

But it’s a ref, Not an out Parameter

I believe this is the cause of the confusion around this issue. ref parameters are both in and out parameters. Noone would mind if we passed a Recipient instead of a Contact as an in parameter so why can’t we do it here? Because ref parameters, like out parameters, have a stricter requirement: you cannot assume the returned type is a more derived type than the parameter’s given type. To do so would not be type safe. The parameter in question must be a ref Contact.

But This Works in Other Languages

This may sometimes work in languages that allow you to subvert type safety (most famously C and C++). It will only work so long as you are lucky about your class layouts or your assumptions remain true. When either class layouts or your assumptions change your code will no longer work.

Here’s an example in C++. I run it in the debugger and it crashes on line 20 due to the type coercion done on line 41. This should not come as a surprise.

 Hope that helps.

In what I would describe as a cockpit I found a something like a display panel which was flashing this message:

Exception in thread “nav” java.lang.ClassCastException: java.lang.Double
at NavigationWaypoint.<init>(NavigationWaypoint.java:10)
at FindWaypoint.FindNextWaypoint(FindWaypoint.java:94)
…

Obviously this sparked my curiosity! In looking around the cockpit I discovered a bit of paper–more like mylar, really–clutched in what for now I’ll call the unfortunate being’s hand. I liberated this piece of paper and found printing on it. I was shocked to see what the paper showed:

What a tragedy! Clearly the author of this code (presumably an inexperienced alien programmer) had errantly designed the instance constructor to take an Object rather than a String. In doing so the coder then needed a typecast in order to coerce the Object to a String in order to set the name field.

I don’t know about most, but on my planet such use of a typecast is a pretty strong clue that the programmer should look around to see why the typecast is necessary; it may be indicative of a design error.

In fact, in this case it is a design error. The NavigationWaypoint class has a name field that contains a string, yet the constructor allows the caller to pass in an object of any type. This unfortunate error and the use of a typecast caused the error to manifest itself during execution of the code; obviously some other code passed in a Double rather than a String, probably a simple coding error. If the constructor had been coded to accept a String this coding error would have been caught at compile time rather than during execution, saving this little fellow the exasperation of debugging on the fly and waking up frozen on a strange planet. If only the constructor had been coded to correctly represent the type’s data….

(This post was inspired by a true story that had nothing to do with navigation.)

using System;
using System.Threading;

class Program
{
    static void Main(string[] args)
    {
        // 'num' lives on the main thread's stack.
        int num = 0;
        Console.WriteLine("initial num=" + num);

        Thread thread = new Thread(
            delegate()
            {
                // Yet this delegate running on a different thread with
                // it's own stack has access to 'num' as well.
                num = 42;
            });

        thread.Start(); // Start the worker thread.
        thread.Join();  // Wait until the worker thread has finished.

        Console.WriteLine("final num=" + num);
    }
}

It’s pretty simple. This program declares and initializes a variable named num, creates and starts a thread that will set num to a different value, then waits for the thread to to complete. In the program’s output we see that num has indeed been set to 42. Here’s the view from the command line:

F:tmp>csc Program.csMicrosoft (R) Visual C# 2005 Compiler version 8.00.50727.42for Microsoft (R) Windows (R) 2005 Framework version 2.0.50727Copyright (C) Microsoft Corporation 2001-2005. All rights reserved.

F:tmp>Program.exeinitial num=0final num=42

Okay, that’s as expected. But wait–num lives on the main thread’s stack. How is it that the second thread we’ve created has access to num? num isn’t on the second thread’s stack and it would be a pretty scary thing if the second thread had direct access to the first thread’s stack. All sorts of mayhem could ensue. Providing access to another thread’s stack isn’t exactly the sort of thing we want to happen in managed code.

Chicanery?

As it turns out, though it appears that num is a stack variable in the main method it is not. What we’re experiencing here is a convenience provided by the compiler. Or rather, a trick. A sleight-of-hand. An illusion. num doesn’t actually live on the stack.

In fact, the compiler has made num a field in a compiler generated class so that it can be made available on the heap to the delegate. Here’s the compiled code, courtesy of .NET Reflector:

[CompilerGenerated]
private sealed class <>c__DisplayClass1
{
      // Methods
      public <>c__DisplayClass1() { }
      public void <main>b__0()
      {
          this.num = 0x2a;
      }

      // Fields
      public int num;
}

The compiler has given our anonymous delegate method a name: it’s <main>b__0() on this compiler generated class. As a member of this class it obviously has access to num. The illusion is completed by fitting out the Main method so that it has an instance of <>c__DisplayClass1 to use:

private static void Main(string[] args)
{
      Program.<>c__DisplayClass1 class1 = new Program.<>c__DisplayClass1();
      class1.num = 0;
      Console.WriteLine("initial num=" + class1.num);

      Thread thread1 = new Thread(new ThreadStart(class1.b__0));
      thread1.Start();
      thread1.Join();

      Console.WriteLine("final num=" + class1.num);
}

So now our Main method has an instance of an object on the heap and can set num to zero since num is a public field on the compiler generated class.

And so, both threads are accessing a variable–not on the stack, but on the heap and accessible to both threads. This helpful behind-the-scenes work by the C# compiler allows us to keep the simplicity of the original code in this example; otherwise, we’d need to implement something like what the compiler has done for us. 

[Edit: Fix up formatting.]

Now, there’s actually something a little sinister going on here… when the app goes away it’s actually exits rather abruptly. There’s no exception thrown, no catch or finally blocks executed–it just (poof) goes away. This isn’t often a problem for me, but if my program modifies some system state while it’s running and needs to revert that state on exit, well, then it is an issue.  

As a simple example consider a program that logs incoming messages. It may have a file open and log messages as they come in. (Or it could alter a registry setting or some other system state to indicate its presence only to revert that setting on exit. Get the idea?) If the app doesn’t terminate normally, how can it clean up after itself?  

First Try: Simple, but broken

To make this a bit more concrete consider this example code. The tricky bit is that when you press Ctrl-C to terminate the program there is no clean up being done. Specifically, the log file isn’t flushed to disk. When I run this program and hit Ctrl-C the log file is left entirely empty. This is because the program hasn’t written enough to the file to flush the file buffer to disk when the program is interrupted. Remember, this program isn’t exiting normally when we use Ctrl-C to interrupt it. In this case, the text “Handling messages.” never appears in the log.  

using System;
using System.IO;
using System.Threading;

namespace Example
{
   class Program
   {
       static void Main(string[] args)
       {
           try
           {
               using (FileStream fs = new FileStream("sample.log", FileMode.Append, FileAccess.Write, FileShare.None))
               using (StreamWriter logWriter = new StreamWriter(fs))
               {
                   Console.WriteLine(
                       "This is the basic program. It doesn't call the clean-up code if you use Ctrl-C, "
                       + "and the log file isn't flushed to disk.");
                   ProcessIncomingMessages(logWriter);
               }
           }
           finally
           {
               Console.WriteLine("Clean-up code invoked.");
           }
       }

       static void ProcessIncomingMessages(TextWriter writer)
       {
           writer.WriteLine("Handling messages.");
           Thread.Sleep(Timeout.Infinite); // Well, pretend it processes messages, okay?
       }
   }
}

Console.CancelKeyPress Event

As you might guess there is a way to handle Ctrl-C and Ctrl-Break and it’s the Console‘s CancelKeyPress event. By providing a delegate you can have the runtime notify you when a “cancel key” has been pressed. Here I’ve  

• Added an anonymous delegate to clean up if Ctrl-C or Ctrl-Break is pressed. As you can see it flushes the writer and closes the file stream.
• I’ve also moved the fs and logWriter variables outside their using statements to allow access from the finally block.

If you read this code from top-to-bottom it may appear unusual that the code in the CancelKeyPress delegate appears to be using logWriter and fs before either variable is assigned a non-null reference. In fact, what is really happening there is that we’re declaring the code for the delegate in line and handing it off to the CancelKeyPress event–it isn’t executed where its position in the source suggests. In this case it’s only executed when Ctrl-C or Ctrl-Break is pressed.  

        static void Main(string[] args)
       {
           FileStream fs = null;
           StreamWriter logWriter = null;
           try
           {
               Console.WriteLine("This shows use of the cancel handler.");
               Console.CancelKeyPress += delegate
               {
                   Console.WriteLine("Clean-up code invoked in CancelKeyPress handler.");
                   if (logWriter != null)
                       logWriter.Flush();
                   if (fs != null)
                       fs.Close();
                   // The application terminates directly after executing this delegate.
               };

               fs = new FileStream("sample.log", FileMode.Append, FileAccess.Write, FileShare.None);
               logWriter = new StreamWriter(fs);
               ProcessIncomingMessages(logWriter);
           }
           finally
           {
               Console.WriteLine("Clean-up code invoked in finally.");
               if (logWriter != null)
                   logWriter.Flush();
               if (fs != null)
                   fs.Close();
           }
       }

Consolidating Clean Up Code

Here I’ve opted to put the clean-up code in one place rather than duplicating it in the finally block. I’ve declared my own delegate type CleanUpMethod so I have a tidy, parameter-less delegate that I can also call from the finally block.  

       delegate void CleanUpMethod();

       static void Main(string[] args)
       {
           FileStream fs = null;
           StreamWriter logWriter = null;

           string cleanUpLocation = "handler.";
           CleanUpMethod cleanUp =
               delegate
               {
                   Console.WriteLine("Clean-up code invoked in " + cleanUpLocation);
                   if (logWriter != null)
                       logWriter.Flush();
                   if (fs != null)
                       fs.Close();
               };

           try
           {
               Console.WriteLine("This shows use of a single, no-param clean-up handler.");
               Console.CancelKeyPress +=
                   delegate
                   {
                       cleanUp();
                       // The application terminates directly after executing this delegate.
                   };

               fs = new FileStream("sample.log", FileMode.Append, FileAccess.Write, FileShare.None);
               logWriter = new StreamWriter(fs);
               ProcessIncomingMessages(logWriter);
           }
           finally
           {
               cleanUpLocation = "finally.";
               cleanUp();
           }
       }

But why put the clean-up code in a delegate instead of it’s own method? One very helpful aspect of using an anonymous delegate in this example is that the delegate code has access to the local variables in this method. If you use Lutz Roeder’s .NET Reflector to take a look at this program you will see the substantial amount of work that the compiler does for you under the covers in order to provide easy access to the local variables from the delegate method:  

private static void Main(string[] args)
{
     ConsoleCancelEventHandler handler1 = null;
     Program.<>c__DisplayClass3 class1 = new Program.<>c__DisplayClass3();
     class1.fs = null;
     class1.logWriter = null;
     class1.cleanUpLocation = "handler.";
     class1.cleanUp = new Program.CleanUpMethod(class1.b__0);
     try
     {
           Console.WriteLine("This shows use of a single, no-param clean-up handler.");
           if (handler1 == null)
           {
                 handler1 = new ConsoleCancelEventHandler(class1.b__1);
           }

           Console.CancelKeyPress += handler1;
           class1.fs = new FileStream("sample.log", FileMode.Append, FileAccess.Write, FileShare.None);
           class1.logWriter = new StreamWriter(class1.fs);
           Program.ProcessIncomingMessages(class1.logWriter);
     }
     finally
     {
           class1.cleanUpLocation = "finally.";
           class1.cleanUp();
     }
}

I'm pleased the C# compiler does so much work just to make my life easier.  

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