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CHAPTER 15 DELEGATES
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Invoking Delegates with Return Values
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If a delegate has a return value and more than one method in its invocation list, the following occurs: The value returned by the last method in the invocation list is the value returned from the delegate invocation. The return values from all the other methods in the invocation list are ignored. For example, the following code declares a delegate that returns an int value. Main creates an object of the delegate and adds two additional methods. It then calls the delegate in the WriteLine statement and prints its return value. Figure 15-9 shows a graphical representation of the code. delegate int MyDel( ); // Declare method with return value. class MyClass { int IntValue = 5; public int Add2() { IntValue += 2; return IntValue;} public int Add3() { IntValue += 3; return IntValue;} } class Program { static void Main( ) { MyClass mc = new MyClass(); MyDel mDel = mc.Add2; // Create and initialize the delegate. mDel += mc.Add3; // Add a method. mDel += mc.Add2; // Add a method. Console.WriteLine("Value: {0}", mDel () ); } } Invoke the delegate and use the return value. This code produces the following output:
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Value: 12
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Figure 15-9. The return value of the last method executed is the value returned by the delegate.
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If a delegate has a reference parameter, the value of the parameter can change upon return from one or more of the methods in the invocation list. When calling the next method in the invocation list, the new value of the parameter not the initial value is the one passed to the next method. For example, the following code invokes a delegate with a reference parameter. Figure 15-10 illustrates the code. delegate void MyDel( ref int X ); class MyClass { public void Add2(ref int x) { x += 2; } public void Add3(ref int x) { x += 3; } static void Main() { MyClass mc = new MyClass(); MyDel mDel = mc.Add2; mDel += mc.Add3; mDel += mc.Add2; int x = 5; mDel(ref x); Console.WriteLine("Value: {0}", x); } } This code produces the following output:
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Value: 12
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CHAPTER 15 DELEGATES
Figure 15-10. The value of a reference parameter can change between calls.
CHAPTER 15 DELEGATES
Anonymous Methods
So far, you ve seen that you can use either static methods or instance methods to instantiate a delegate. In either case, the method itself can be called explicitly from other parts of the code, and, of course, must be a member of some class or struct. What if, however, the method is used only one time to instantiate the delegate In that case, other than the syntactic requirement for creating the delegate, there is no real need for a separate, named method. Anonymous methods allow you to dispense with the separate, named method. An anonymous method is a method that is declared inline, at the point of instantiating a delegate. For example, Figure 15-11 shows two versions of the same class. The version on the left declares and uses a method named Add20. The version on the right uses an anonymous method instead. The nonshaded code of both versions is identical.
Figure 15-11. Comparing a named method and an anonymous method Both sets of code in Figure 15-11 produce the following output: 25 26
Using Anonymous Methods
You can use an anonymous method in the following places: As an initializer expression when declaring a delegate variable. On the right-hand side of an assignment statement when combining delegates. On the right-hand side of an assignment statement adding a delegate to an event. 16 covers events.
CHAPTER 15 DELEGATES
Syntax of Anonymous Methods
The syntax of an anonymous method expression includes the following components: The type keyword delegate The parameter list, which can be omitted if the statement block doesn t use any parameters The statement block, which contains the code of the anonymous method Parameter Keyword list delegate ( Parameters )
Statement block { ImplementationCode }
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