Delegate Class

 

Represents a delegate, which is a data structure that refers to a static method or to a class instance and an instance method of that class.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)


<SerializableAttribute>
<ClassInterfaceAttribute(ClassInterfaceType.AutoDual)>
<ComVisibleAttribute(True)>
Public MustInherit Class Delegate
	Implements ICloneable, ISerializable

NameDescription
System_CAPS_protmethodDelegate(Object, String)

Initializes a delegate that invokes the specified instance method on the specified class instance.

System_CAPS_protmethodDelegate(Type, String)

Initializes a delegate that invokes the specified static method from the specified class.

NameDescription
System_CAPS_pubpropertyMethod

Gets the method represented by the delegate.

System_CAPS_pubpropertyTarget

Gets the class instance on which the current delegate invokes the instance method.

NameDescription
System_CAPS_pubmethodClone()

Creates a shallow copy of the delegate.

System_CAPS_pubmethodSystem_CAPS_staticCombine(Delegate, Delegate)

Concatenates the invocation lists of two delegates.

System_CAPS_pubmethodSystem_CAPS_staticCombine(Delegate())

Concatenates the invocation lists of an array of delegates.

System_CAPS_protmethodCombineImpl(Delegate)

Concatenates the invocation lists of the specified multicast (combinable) delegate and the current multicast (combinable) delegate.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, MethodInfo)

Creates a delegate of the specified type to represent the specified static method.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, MethodInfo, Boolean)

Creates a delegate of the specified type to represent the specified static method, with the specified behavior on failure to bind.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Object, MethodInfo)

Creates a delegate of the specified type that represents the specified static or instance method, with the specified first argument.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Object, MethodInfo, Boolean)

Creates a delegate of the specified type that represents the specified static or instance method, with the specified first argument and the specified behavior on failure to bind.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Object, String)

Creates a delegate of the specified type that represents the specified instance method to invoke on the specified class instance.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Object, String, Boolean)

Creates a delegate of the specified type that represents the specified instance method to invoke on the specified class instance with the specified case-sensitivity.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Object, String, Boolean, Boolean)

Creates a delegate of the specified type that represents the specified instance method to invoke on the specified class instance, with the specified case-sensitivity and the specified behavior on failure to bind.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Type, String)

Creates a delegate of the specified type that represents the specified static method of the specified class.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Type, String, Boolean)

Creates a delegate of the specified type that represents the specified static method of the specified class, with the specified case-sensitivity.

System_CAPS_pubmethodSystem_CAPS_staticCreateDelegate(Type, Type, String, Boolean, Boolean)

Creates a delegate of the specified type that represents the specified static method of the specified class, with the specified case-sensitivity and the specified behavior on failure to bind.

System_CAPS_pubmethodDynamicInvoke(Object())

Dynamically invokes (late-bound) the method represented by the current delegate.

System_CAPS_protmethodDynamicInvokeImpl(Object())

Dynamically invokes (late-bound) the method represented by the current delegate.

System_CAPS_pubmethodEquals(Object)

Determines whether the specified object and the current delegate are of the same type and share the same targets, methods, and invocation list.(Overrides Object.Equals(Object).)

System_CAPS_protmethodFinalize()

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)

System_CAPS_pubmethodGetHashCode()

Returns a hash code for the delegate.(Overrides Object.GetHashCode().)

System_CAPS_pubmethodGetInvocationList()

Returns the invocation list of the delegate.

System_CAPS_protmethodGetMethodImpl()

Gets the static method represented by the current delegate.

System_CAPS_pubmethodGetObjectData(SerializationInfo, StreamingContext)

Not supported.

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_protmethodMemberwiseClone()

Creates a shallow copy of the current Object.(Inherited from Object.)

System_CAPS_pubmethodSystem_CAPS_staticRemove(Delegate, Delegate)

Removes the last occurrence of the invocation list of a delegate from the invocation list of another delegate.

System_CAPS_pubmethodSystem_CAPS_staticRemoveAll(Delegate, Delegate)

Removes all occurrences of the invocation list of a delegate from the invocation list of another delegate.

System_CAPS_protmethodRemoveImpl(Delegate)

Removes the invocation list of a delegate from the invocation list of another delegate.

System_CAPS_pubmethodToString()

Returns a string that represents the current object.(Inherited from Object.)

NameDescription
System_CAPS_puboperatorSystem_CAPS_staticEquality(Delegate, Delegate)

Determines whether the specified delegates are equal.

System_CAPS_puboperatorSystem_CAPS_staticInequality(Delegate, Delegate)

Determines whether the specified delegates are not equal.

NameDescription
System_CAPS_pubmethodGetMethodInfo()

Gets an object that represents the method represented by the specified delegate.(Defined by RuntimeReflectionExtensions.)

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.

System_CAPS_noteNote

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#, Visual Basic, or Visual C++, 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 common language runtime provides each delegate type with BeginInvoke and EndInvoke methods, to enable asynchronous invocation of the delegate. For more information about these methods, see Calling Synchronous Methods Asynchronously.

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:

  • 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 more information on delegate binding, see the CreateDelegate(Type, Object, MethodInfo, Boolean) method overload.

System_CAPS_noteNote

In the .NET Framework versions 1.0 and 1.1, a delegate can represent a method only if the signature of the method exactly matches the signature specified by the delegate type. Thus, only the first and third bullets in the preceding list are supported, and the first bullet requires an exact type match.

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 null. A combining operation returns null 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.

System_CAPS_noteNote

Managed languages use the Combine and Remove methods to implement delegate operations. Examples include the AddHandler and RemoveHandler statements in Visual Basic and the += and -= operators on delegate types in C#.

Starting with the .NET Framework 4, generic delegate types can have variant type parameters. Contravariant type parameters can be used as parameter types of the delegate, and a covariant type parameter can be used as the return type. This feature allows generic delegate types that are constructed from the same generic type definition to be assignment-compatible if their type arguments are reference types with an inheritance relationship, as explained in Covariance and Contravariance in Generics.

System_CAPS_noteNote

Generic delegates that are assignment-compatible because of variance are not necessarily combinable. To be combinable, the types must match exactly. For example, suppose that a class named Derived is derived from a class named Base. A delegate of type Action<Base> (Action(Of Base) in Visual Basic) can be assigned to a variable of type Action<Derived>, but the two delegates cannot be combined because the types do not match exactly.

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 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.

Imports System

Public Class SamplesDelegate

   ' Declares a delegate for a method that takes in an int and returns a String.
   Delegate Function myMethodDelegate(myInt As Integer) As [String]

   ' Defines some methods to which the delegate can point.
   Public Class mySampleClass

      ' Defines an instance method.
      Public Function myStringMethod(myInt As Integer) As [String]
         If myInt > 0 Then
            Return "positive"
         End If
         If myInt < 0 Then
            Return "negative"
         End If
         Return "zero"
      End Function 'myStringMethod

      ' Defines a static method.
      Public Shared Function mySignMethod(myInt As Integer) As [String]
         If myInt > 0 Then
            Return "+"
         End If
         If myInt < 0 Then
            Return "-"
         End If
         Return ""
      End Function 'mySignMethod
   End Class 'mySampleClass

   Public Shared Sub Main()

      ' Creates one delegate for each method. For the instance method, an
      ' instance (mySC) must be supplied. For the Shared method, the
      ' method name is qualified by the class name.
      Dim mySC As New mySampleClass()
      Dim myD1 As New myMethodDelegate(AddressOf mySC.myStringMethod)
      Dim myD2 As New myMethodDelegate(AddressOf mySampleClass.mySignMethod)

      ' Invokes the delegates.
      Console.WriteLine("{0} is {1}; use the sign ""{2}"".", 5, myD1(5), myD2(5))
      Console.WriteLine("{0} is {1}; use the sign ""{2}"".", - 3, myD1(- 3), myD2(- 3))
      Console.WriteLine("{0} is {1}; use the sign ""{2}"".", 0, myD1(0), myD2(0))

   End Sub 'Main

End Class 'SamplesDelegate 


'This code produces the following output:
' 
'5 is positive; use the sign "+".
'-3 is negative; use the sign "-".
'0 is zero; use the sign "".


Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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