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C# s Syntactical Sugar for Delegates
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Most programmers find working with delegates to be cumbersome because the syntax is so strange . For example, take this line of code:
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button1.Click += new EventHandler(button1_Click);
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where button1_Click is a method that looks something like this:
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void button1_Click(Object sender, EventArgs e) { // Do something, the button was clicked... }
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The idea behind the first line of code is to register the address of the button1_Click method with a button control so that when the button is clicked, the method will be called . To most programmers, it feels quite unnatural to construct an EventHandler delegate object just to specify the address of the button1_Click method . However, constructing the EventHandler delegate object is required for the CLR because this object provides a wrapper that ensures that the method can be called only in a type-safe fashion . The wrapper also allows the calling of instance methods and chaining . Unfortunately, most programmers don t want to think about these details . Programmers would prefer to write the code above as follows:
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button1.Click += button1_Click;
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Fortunately, Microsoft s C# compiler offers programmers some syntax shortcuts when working with delegates . I ll explain all of these shortcuts in this section . One last point before we begin: what I m about to describe really boils down to C# syntactical sugar; these new syntax shortcuts are really just giving programmers an easier way to produce the IL that must be generated so that the CLR and other programming languages can work with delegates . This also means that what I m about to describe is specific to C#; other compilers might not offer the additional delegate syntax shortcuts .
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Syntactical Shortcut #1: No Need to Construct a Delegate Object
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As demonstrated already, C# allows you to specify the name of a callback method without having to construct a delegate object wrapper . Here is another example:
internal sealed class AClass { public static void CallbackWithoutNewingADelegateObject() { ThreadPool.QueueUserWorkItem(SomeAsyncTask, 5); } private static void SomeAsyncTask(Object o) { Console.WriteLine(o); } }
Here, the ThreadPool class s static QueueUserWorkItem method expects a reference to a WaitCallback delegate object that contains a reference to the SomeAsyncTask method . Since the C# compiler is capable of inferring this on its own, it allows me to omit code that constructs the WaitCallback delegate object, making the code much more readable and understandable . Of course, when the code is compiled, the C# compiler does produce IL that does, in fact, new up the WaitCallback delegate object we just got a syntactical shortcut .
Syntactical Shortcut #2: No Need to Define a Callback Method
In the code above, the name of the callback method, SomeAsyncTask, is passed to the ThreadPool s QueueUserWorkItem method . C# allows you to write the code for the callback
17 Delegates
method inline so it doesn t have to be written inside its very own method . For example, the code above could be rewritten as follows:
internal sealed class AClass { public static void CallbackWithoutNewingADelegateObject() { ThreadPool.QueueUserWorkItem( obj => Console.WriteLine(obj), 5); } }
Notice that the first argument to the QueueUserWorkItem method is code (which I italicized)! More formally, the italicized code is called a C# lambda expression, and it is easy to detect due to the use of C#'s lambda expression operator: => . You may use a lambda expression in your code where the compiler would normally expect to see a delegate . And, when the compiler sees the use of this lambda expression, the compiler automatically defines a new private method in the class (AClass, in this example) . This new method is called an anonymous function because the compiler creates the name of the method for you automatically, and normally, you wouldn t know its name . However, you could use a tool such as ILDasm .exe to examine the compiler-generated code . After I wrote the code above and compiled it, I was able to see, by using ILDasm .exe, that the C# compiler decided to name this method <CallbackWithoutNewingADelegateObject>b__0 and ensured that this method took a single Object argument and returned void . The compiler chose to start the method name with a < sign because in C#, an identifier cannot contain a < sign; this ensures that you will not accidentally define a method that coincides with the name the compiler has chosen for you . Incidentally, while C# forbids identifiers to contain a < sign, the CLR allows it, and that is why this works . Also, note that while you could access the method via reflection by passing the method name as a string, the C# language specification states that there is no guarantee of how the compiler generates the name . For example, each time you compile the code, the compiler could produce a different name for the method . Using ILDasm .exe, you might also notice that the C# compiler applies the System.Runtime.CompilerServices.CompilerGeneratedAttribute attribute to this method to indicate to various tools and utilities that this method was produced by a compiler as opposed to a programmer . The code to the right of the => operator is then placed in this compiler-generated method . Note When writing a lambda expression, there is no way to apply your own custom attribute to
the compiler-generated method . Furthermore, you cannot apply any method modifiers (such as unsafe) to the method . But this is usually not a problem because anonymous methods generated by the compiler always end up being private, and the method is either static or nonstatic depending on whether the method accesses any instance members . So there is no need to apply modifiers such as public, protected, internal, virtual, sealed, override, or abstract to the method .
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