Delegates in Silverlight

Dinesh Beniwal

Delegates are reference types that serve a purpose similar to that of function pointers in C++. They are used for event handlers and callback functions in the .NET Framework. Unlike function pointers, delegates are secure, verifiable, and type safe. A delegate type can represent any instance method or static method that has a compatible signature.

A parameter of a delegate is compatible with the corresponding parameter of a method if the type of the delegate parameter is more restrictive than the type of the method parameter, because this guarantees that an argument passed to the delegate can be passed safely to the method.

Similarly, the return type of a delegate is compatible with the return type of a method if the return type of the method is more restrictive than the return type of the delegate, because this guarantees that the return value of the method can be cast safely to the return type of the delegate.

For example, a delegate that has a parameter of type IEnumerable and a return type of Object can represent a method that has a parameter of type Object and a return value of type IEnumerable. For more information and example code, see Delegate..::.CreateDelegate(Type, Object, MethodInfo).

A delegate is said to be bound to the method it represents. In addition to being bound to the method, a delegate can be bound to an object. The object represents the first parameter of the method, and is passed to the method every time the delegate is invoked. If the method is an instance method, the bound object is passed as the implicit this parameter (Me in Visual Basic); if the method is static, the object is passed as the first formal parameter of the method, and the delegate signature must match the remaining parameters. For more information and example code, see Delegate.

All delegates inherit from MulticastDelegate, which inherits from Delegate. The C#, Visual Basic, and C++ languages do not allow inheritance from these types. Instead, they provide keywords for declaring delegates.

Because delegates inherit from MulticastDelegate, a delegate has an invocation list, which is a list of methods that the delegate represents and that are executed when the delegate is invoked. All methods in the list receive the arguments supplied when the delegate is invoked.

Note:
The return value is not defined for a delegate that has more than one method in its invocation list, even if the delegate has a return type.

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In many cases, such as with callback methods, a delegate represents only one method, and the only actions you have to take are creating the delegate and invoking it.

For delegates that represent multiple methods, the .NET Framework provides methods of the Delegate and MulticastDelegate delegate classes to support operations such as adding a method to a delegate's invocation list (the Delegate..::.Combine method), removing a method (the Delegate..::.Remove method), and getting the invocation list (the Delegate..::.GetInvocationList method).

Note:
It is not necessary to use these methods for event-handler delegates in C#, C++, and Visual Basic, because these languages provide syntax for adding and removing event handlers.