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15.Delegates

A delegate declaration defines a class that is derived from the class System.Delegate. A delegate instance encapsulates an invocation list, which is a list of one or more methods, each of which is referred to as a callable entity. For instance methods, a callable entity consists of an instance and a method on that instance. For static methods, a callable entity consists of just a method. Invoking a delegate instance with an appropriate set of arguments causes each of the delegate’s callable entities to be invoked with the given set of arguments.

An interesting and useful property of a delegate instance is that it does not know or care about the classes of the methods it encapsulates; all that matters is that those methods be compatible (§15.1) with the delegate’s type. This makes delegates perfectly suited for “anonymous” invocation.

15.1Delegate declarations

It is a compile-time error for the same modifier to appear multiple times in a delegate declaration.

The new modifier is only permitted on delegates declared within another type, in which case it specifies that such a delegate hides an inherited member by the same name, as described in §10.3.4.

The public, protected, internal, and private modifiers control the accessibility of the delegate type. Depending on the context in which the delegate declaration occurs, some of these modifiers may not be permitted (§3.5.1).

The delegate’s type name is identifier.

The optional formal-parameter-list specifies the parameters of the delegate, and return-type indicates the return type of the delegate.

The optional type-parameter-list specifies the type parameters to the delegate itself.

Delegate types in C# are name equivalent, not structurally equivalent. Specifically, two different delegate types that have the same parameter lists and return type are considered different delegate types. However, instances of two distinct but structurally equivalent delegate types may compare as equal (§7.9.8).

For example:

delegate int D1(int i, double d);

class A
{
public static int M1(int a, double b) {...}
}

class B
{

public static int M1(int f, double g) {...}

public static void M2(int k, double l) {...}

public static int M3(int g) {...}

public static void M4(int g) {...}
}

The delegate types D1 and D2 are both compatible with the methods A.M1 and B.M1, since they have the same return type and parameter list; however, these delegate types are two different types, so they are not interchangeable. The delegate types D1 and D2 are incompatible with the methods B.M2, B.M3, and B.M4, since they have different return types or parameter lists.

Like other generic type declarations, type arguments must be given to create a constructed delegate type. The parameter types and return type of a constructed delegate type are created by substituting, for each type parameter in the delegate declaration, the corresponding type argument of the constructed delegate type. The resulting return type and parameter types are used in determining what methods are compatible with a constructed delegate type. For example:

delegate bool Predicate(T value);

class X
{
static bool F(int i) {...}

static bool G(string s) {...}

The delegate type Predicate is compatible with the method X.F and the delegate type Predicate is compatible with the method X.G.

The only way to declare a delegate type is via a delegate-declaration. A delegate type is a class type that is derived from System.Delegate. Delegate types are implicitly sealed, so it is not permissible to derive any type from a delegate type. It is also not permissible to derive a non-delegate class type from System.Delegate. Note that System.Delegate is not itself a delegate type; it is a class type from which all delegate types are derived.

C# provides special syntax for delegate instantiation and invocation. Except for instantiation, any operation that can be applied to a class or class instance can also be applied to a delegate class or instance, respectively. In particular, it is possible to access members of the System.Delegate type via the usual member access syntax.

The set of methods encapsulated by a delegate instance is called an invocation list. When a delegate instance is created (§15.2) from a single method, it encapsulates that method, and its invocation list contains only one entry. However, when two non-null delegate instances are combined, their invocation lists are concatenated—in the order left operand then right operand—to form a new invocation list, which contains two or more entries.

Delegates are combined using the binary + (§7.7.4) and += operators (§7.16.2). A delegate can be removed from a combination of delegates, using the binary - (§7.7.5) and -= operators (§7.16.2). Delegates can be compared for equality (§7.9.8).

The following example shows the instantiation of a number of delegates, and their corresponding invocation lists:

delegate void D(int x);

class C
{
public static void M1(int i) {...}

public static void M2(int i) {...}

}

class Test

}

When cd1 and cd2 are instantiated, they each encapsulate one method. When cd3 is instantiated, it has an invocation list of two methods, M1 and M2, in that order. cd4’s invocation list contains M1, M2, and M1, in that order. Finally, cd5’s invocation list contains M1, M2, M1, M1, and M2, in that order. For more examples of combining (as well as removing) delegates, see §15.4.