Part III Essential Types in Visual C#.NET

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Part III Essential Types
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using Delegates to Call Back Static Methods
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Now that you understand how the Counter method is designed and how it works, let s see how to use delegates to call back static methods . The StaticDelegateDemo method that appears in the previous code sample is the focus of this section . The StaticDelegateDemo method calls the Counter method, passing null in the third parameter, which corresponds to Counter s fb parameter . Because Counter s fb parameter receives null, each item is processed without calling any callback method . Next, the StaticDelegateDemo method calls Counter a second time, passing a newly constructed Feedback delegate object in the third parameter of the method call . This delegate object is a wrapper around a method, allowing the method to be called back indirectly via the wrapper . In this example, the name of the static method, Program.FeedbackToConsole, is passed to the Feedback type s constructor, indicating that it is the method to be wrapped . The reference returned from the new operator is passed to Counter as its third parameter . Now when Counter executes, it will call the Program type s static FeedbackToConsole method for each item in the series . FeedbackToConsole simply writes a string to the console indicating the item being processed . Note The FeedbackToConsole method is defined as private inside the Program type, but
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the Counter method is able to call Program s private method . In this case, you might not expect a problem because both Counter and FeedbackToConsole are defined in the same type . However, this code would work just fine even if the Counter method was defined in another type . In short, it is not a security or accessibility violation for one type to have code that calls another type s private member via a delegate as long as the delegate object is created by code that has ample security/accessibility .
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The third call to Counter in the StaticDelegateDemo method is almost identical to the second call . The only difference is that the Feedback delegate object wraps the static Program.FeedbackToMsgBox method . FeedbackToMsgBox builds a string indicating the item being processed . This string is then displayed in a message box . Everything in this example is type-safe . For instance, when constructing a Feedback delegate object, the compiler ensures that the signatures of Program s FeedbackToConsole and FeedbackToMsgBox methods are compatible with the signature defined by the Feedback delegate . Specifically, both methods must take one argument (an Int32), and both methods must have the same return type (void) . If FeedbackToConsole had been defined like this:
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private static Boolean FeedbackToConsole(String value) { ... }
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the C# compiler wouldn t compile the code and would issue the following error: "error
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CS0123: No overload for 'FeedbackToConsole' matches delegate 'Feedback'."
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Both C# and the CLR allow for covariance and contra-variance of reference types when binding a method to a delegate . Covariance means that a method can return a type that is derived from the delegate s return type . Contra-variance means that a method can take a parameter that is a base of the delegate s parameter type . For example, given a delegate defined like this:
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delegate Object MyCallback(FileStream s);
it is possible to construct an instance of this delegate type bound to a method that is prototyped like this:
String SomeMethod(Stream s);
Here, SomeMethod s return type (String) is a type that is derived from the delegate s return type (Object); this covariance is allowed . SomeMethod s parameter type (Stream) is a type that is a base class of the delegate s parameter type (FileStream); this contra-variance is allowed . Note that covariance and contra-variance are supported only for reference types, not for value types or for void . So, for example, I cannot bind the following method to the MyCallback delegate:
Int32 SomeOtherMethod(Stream s);
Even though SomeOtherMethod s return type (Int32) is derived from MyCallback s return type (Object), this form of covariance is not allowed because Int32 is a value type . Obviously, the reason why value types and void cannot be used for covariance and contra-variance is because the memory structure for these things varies, whereas the memory structure for reference types is always a pointer . Fortunately, the C# compiler will produce an error if you attempt to do something that is not supported .
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