Delegate Class

The Delegate class is the base class for delegate types. However, only the system and compilers can derive explicitly from the Delegate class or from the MulticastDelegate class. It is also not permissible to derive a new type from a delegate type. The Delegate class is not considered a delegate type; it is a class used to derive delegate types.

Most languages implement a delegate keyword, and compilers for those languages are able to derive from the MulticastDelegate class; therefore, users should use the delegate keyword provided by the language.

Note:
The common language runtime provides an Invoke method for each delegate type, with the same signature as the delegate. You do not have to call this method explicitly from C# or Visual Basic, because the compilers call it automatically. The Invoke method is useful in reflection when you want to find the signature of the delegate type.

The declaration of a delegate type establishes a contract that specifies the signature of one or more methods. A delegate is an instance of a delegate type that has references to one of the following:

• An instance method of a type and a target object assignable to that type.

• An instance method of a type, with the hidden this parameter exposed in the formal parameter list. The delegate is said to be an open instance delegate.

• A static method.

• A static method and a target object assignable to the first parameter of the method. The delegate is said to be closed over its first argument.

For examples of delegate binding in these scenarios, see the CreateDelegate(Type, Object, MethodInfo, Boolean) method overload.

When a delegate represents an instance method closed over its first argument (the most common case), the delegate stores a reference to the method's entry point and a reference to an object, called the target, which is of a type assignable to the type that defined the method. When a delegate represents an open instance method, it stores a reference to the method's entry point. The delegate signature must include the hidden this parameter in its formal parameter list; in this case, the delegate does not have a reference to a target object, and a target object must be supplied when the delegate is invoked.

When a delegate represents a static method, the delegate stores a reference to the method's entry point. When a delegate represents a static method closed over its first argument, the delegate stores a reference to the method's entry point and a reference to a target object assignable to the type of the method's first argument. When the delegate is invoked, the first argument of the static method receives the target object.

The invocation list of a delegate is an ordered set of delegates in which each element of the list invokes exactly one of the methods represented by the delegate. An invocation list can contain duplicate methods. During an invocation, methods are invoked in the order in which they appear in the invocation list. A delegate attempts to invoke every method in its invocation list; duplicates are invoked once for each time they appear in the invocation list. Delegates are immutable; once created, the invocation list of a delegate does not change.

Delegates are referred to as multicast, or combinable, because a delegate can invoke one or more methods and can be used in combining operations.

Combining operations, such as Combine and Remove, do not alter existing delegates. Instead, such an operation returns a new delegate that contains the results of the operation, an unchanged delegate, or nullptr. A combining operation returns nullptr when the result of the operation is a delegate that does not reference at least one method. A combining operation returns an unchanged delegate when the requested operation has no effect.

If an invoked method throws an exception, the method stops executing, the exception is passed back to the caller of the delegate, and remaining methods in the invocation list are not invoked. Catching the exception in the caller does not alter this behavior.

When the signature of the methods invoked by a delegate includes a return value, the delegate returns the return value of the last element in the invocation list. When the signature includes a parameter that is passed by reference, the final value of the parameter is the result of every method in the invocation list executing sequentially and updating the parameter's value.

The closest equivalent of a delegate in C or C++ is a function pointer. A delegate can represent a static method or an instance method. When the delegate represents an instance method, the delegate stores not only a reference to the method's entry point, but also a reference to the class instance. Unlike function pointers, delegates are object-oriented and type safe.

The BeginInvoke method that is automatically defined on all delegate types always throws a NotSupportedException, so you cannot use it to make asynchronous method calls on thread pool threads. You can use the Dispatcher::BeginInvoke(Delegate, array<Object>) method overload to execute a method on the thread that a System.Windows.Threading::Dispatcher object is associated with, but this technique uses late binding and does not employ thread pool threads.

Platform Notes

The following example shows how to define a delegate named myMethodDelegate. Instances of this delegate are created for an instance method and a static method of the nested mySampleClass class. The delegate for the instance method requires an instance of mySampleClass. The mySampleClass instance is saved in a variable named mySC.

Note:
To run this example, see Building examples that have static TextBlock controls for Windows Phone 8.

Windows Phone OS

Windows Phone

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