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Using Delegates to Call Back Static Methods
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Now that you understand how the Set type is designed and how it works, let s see how to use delegates to call back static methods. The StaticCallbacks method that appears in the previous code sample is the focus of this section. The StaticCallbacks method begins by constructing a Set object, telling it to create an array of five objects. Then ProcessItems is called, passing it null for its feedback parameter. ProcessItems represents a method that performs some action for every item managed by the Set. Because the feedback parameter is null in this example, each item is processed without calling any callback methods. For the second call to ProcessItems, a new Set.Feedback delegate object is constructed. This delegate object is a wrapper around a method, allowing that method to be called back indirectly via the wrapper. To the Feedback type s constructor, the name of a static method, App.FeedbackToConsole in this example, is passed; this indicates the method to be wrapped. The reference returned from the new operator is then passed to ProcessItems. Now, when ProcessItems executes, it will call the App type s static FeedbackToConsole method for each item in the set. FeedbackToConsole simply writes a string to the console indicating the item being processed and the item s value. Note The FeedbackToConsole method is defined as private inside the App type, but the Set type s ProcessItems method is able to call App s private method. No security problem results here because App s code explicitly decided to return a delegate wrapper over a private method. The third call to ProcessItems is almost identical to the second call. The only difference is that the Feedback delegate object wraps the static App.FeedbackToMsgBox method. FeedbackToMsgBox builds a string indicating the item being processed and the item s value. This string is then displayed in a message box.
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The fourth and final call to ProcessItems demonstrates how delegates can be linked together to form a chain. In this example, a reference variable to a Feedback delegate object, fb, is created and initialized to null. This variable points to the head of a linked list of delegates. A value of null indicates that there are no nodes in the linked list. Then a Feedback delegate object that wraps a call to App s FeedbackToConsole method is constructed. The C# += operator is used to append this object to the linked list referred to by fb. The fb variable now refers to the head of the linked list. Finally, another Feedback delegate object is constructed that wraps a call to App s FeedbackToMsgBox method. Again, the C# += operator is used to append this object to the linked list and fb is updated to refer to the new head of the linked list. Now, when ProcessItems is called, it is passed the head of the linked list of Feedback delegates. Inside ProcessItems, the line of code that calls the callback method actually ends up calling all the callback methods wrapped by the delegate objects in the linked list. In other words, for each item being iterated, FeedbackToConsole will be called immediately followed by FeedbackToMsgBox. I ll explain exactly how a delegate chain works later in this chapter. Everything in this example is type safe. For instance, when constructing a Feedback delegate object, the compiler ensures that App s FeedbackToConsole and FeedbackToMsgBox methods have the exact prototype as defined by the Feed back delegate; that is, both methods must take three parameters (Object and two Int32s) and both methods must have the same return type (void). What would have happened if FeedbackToConsole had been prototyped like this
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void FeedbackToConsole(Object, Int32, Int32, String s) { }
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The C# compiler wouldn t compile the code and would issue the following error: error CS0123: The signature of method App.FeedbackToConsole() does not match this delegate type .
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