Delegate.CreateDelegate Method (Type, Object, MethodInfo)

 

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

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

Public Shared Function CreateDelegate (
	type As Type,
	firstArgument As Object,
	method As MethodInfo
) As Delegate

Parameters

type
Type: System.Type

The Type of delegate to create.

firstArgument
Type: System.Object

The object to which the delegate is bound, or null to treat method as static (Shared in Visual Basic).

method
Type: System.Reflection.MethodInfo

The MethodInfo describing the static or instance method the delegate is to represent.

Return Value

Type: System.Delegate

A delegate of the specified type that represents the specified static or instance method.

Exception Condition
ArgumentNullException

type is null.

-or-

method is null.

ArgumentException

type does not inherit MulticastDelegate.

-or-

type is not a RuntimeType. See Runtime Types in Reflection.

-or-

method cannot be bound.

-or-

method is not a RuntimeMethodInfo. See Runtime Types in Reflection.

MissingMethodException

The Invoke method of type is not found.

MethodAccessException

The caller does not have the permissions necessary to access method.

Calling this method overload is equivalent to calling the CreateDelegate(Type, Object, MethodInfo, Boolean) method overload and specifying true for throwOnBindFailure. These two overloads provide the most flexible way to create delegates. You can use them to create delegates for either static or instance methods, and optionally to specify the first argument.

System_CAPS_noteNote

If you do not supply a first argument, use the CreateDelegate(Type, MethodInfo) method overload for better performance.

The delegate type and the method must have compatible return types. That is, the return type of method must be assignable to the return type of type.

If firstArgument is supplied, it is passed to method every time the delegate is invoked; firstArgument is said to be bound to the delegate, and the delegate is said to be closed over its first argument. If method is static (Shared in Visual Basic), the argument list supplied when invoking the delegate includes all parameters except the first; if method is an instance method, then firstArgument is passed to the hidden instance parameter (represented by this in C#, or by Me in Visual Basic).

If firstArgument is supplied, the first parameter of method must be a reference type, and firstArgument must be compatible with that type.

System_CAPS_importantImportant

If method is static (Shared in Visual Basic) and its first parameter is of type Object or ValueType, then firstArgument can be a value type. In this case firstArgument is automatically boxed. Automatic boxing does not occur for any other arguments, as it would in a C# or Visual Basic function call.

If firstArgument is a null reference and method is an instance method, the result depends on the signatures of the delegate type type and of method:

  • If the signature of type explicitly includes the hidden first parameter of method, the delegate is said to represent an open instance method. When the delegate is invoked, the first argument in the argument list is passed to the hidden instance parameter of method.

  • If the signatures of method and type match (that is, all parameter types are compatible), then the delegate is said to be closed over a null reference. Invoking the delegate is like calling an instance method on a null instance, which is not a particularly useful thing to do.

If firstArgument is a null reference and method is static, the result depends on the signatures of the delegate type type and of method:

  • If the signature of method and type match (that is, all parameter types are compatible), the delegate is said to represent an open static method. This is the most common case for static methods. In this case, you can get slightly better performance by using the CreateDelegate(Type, MethodInfo) method overload.

  • If the signature of type begins with the second parameter of method and the rest of the parameter types are compatible, then the delegate is said to be closed over a null reference. When the delegate is invoked, a null reference is passed to the first parameter of method.

System_CAPS_noteNote

Starting with the .NET Framework 2.0 Service Pack 1, this method can be used to access non-public methods if the caller has been granted ReflectionPermission with the ReflectionPermissionFlag.RestrictedMemberAccess flag and if the grant set of the non-public methods is restricted to the caller’s grant set, or a subset thereof. (See Security Considerations for Reflection.)

To use this functionality, your application should target the .NET Framework 3.5 or later.

The parameter types and return type of a delegate must be compatible with the parameter types and return type of the method the delegate represents; the types do not have to match exactly.

System_CAPS_noteNote

In the .NET Framework version 1.0 and 1.1, the types must match exactly.

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 with a parameter of type Hashtable and a return type of Object can represent a method with a parameter of type Object and a return value of type Hashtable.

Another useful way to think of the flexibility provided by this overload of CreateDelegate is that any given delegate can represent four different combinations of method signature and method kind (static versus instance). Consider a delegate type D with one argument of type C. The following describes the methods D can represent, ignoring the return type since it must match in all cases:

  • D can represent any instance method that has exactly one argument of type C, regardless of what type the instance method belongs to. When CreateDelegate is called, firstArgument is an instance of the type method belongs to, and the resulting delegate is said to be closed over that instance. (Trivially, D can also be closed over a null reference if firstArgument is a null reference.)

  • D can represent an instance method of C that has no arguments. When CreateDelegate is called, firstArgument is a null reference. The resulting delegate represents an open instance method, and an instance of C must be supplied each time it is invoked.

  • D can represent a static method that takes one argument of type C, and that method can belong to any type. When CreateDelegate is called, firstArgument is a null reference. The resulting delegate represents an open static method, and an instance of C must be supplied each time it is invoked.

  • D can represent a static method that belongs to type F and has two arguments, of type F and type C. When CreateDelegate is called, firstArgument is an instance of F. The resulting delegate represents a static method that is closed over that instance of F. Note that in the case where F and C are the same type, the static method has two arguments of that type. (In this case, D is closed over a null reference if firstArgument is a null reference.)

This section contains three code examples. The first example demonstrates the four kinds of delegates that can be created: closed over an instance method, open over an instance method, open over a static method, and closed over a static method.

The second code example demonstrates compatible parameter types and return types.

The third code example defines a single delegate type, and shows all the methods that delegate type can represent.

Example 1

The following code example demonstrates the four ways a delegate can be created using this overload of the CreateDelegate method.

System_CAPS_noteNote

There are two overloads of the CreateDelegate method that specify firstArgument and a MethodInfo; their functionality is the same except that one allows you to specify whether to throw on failure to bind, and the other always throws. This code example uses both overloads.

The example declares a class C with a static method M2 and an instance method M1, and three delegate types: D1 takes an instance of C and a string, D2 takes a string, and D3 has no arguments.

A second class named Example contains the code that creates the delegates.

  • A delegate of type D2, closed over an instance of C, is created for the instance method M1. It is invoked with different strings, to show that the bound instance of C is always used.

  • A delegate of type D1, representing an open instance method, is created for the instance method M1. An instance must be passed when the delegate is invoked.

  • A delegate of type D2, representing an open static method, is created for the static method M2.

  • Finally, a delegate of type D3, closed over a string, is created for the static method M2. The method is invoked to show that it uses the bound string.

Imports System
Imports System.Reflection
Imports System.Security.Permissions

' Declare three delegate types for demonstrating the combinations
' of Shared versus instance methods and open versus closed
' delegates.
'
Public Delegate Sub D1(ByVal c As C, ByVal s As String) 
Public Delegate Sub D2(ByVal s As String) 
Public Delegate Sub D3() 

' A sample class with an instance method and a Shared method.
'
Public Class C
    Private id As Integer
    Public Sub New(ByVal id As Integer) 
        Me.id = id
    End Sub 'NewNew

    Public Sub M1(ByVal s As String) 
        Console.WriteLine("Instance method M1 on C:  id = {0}, s = {1}", _
            Me.id, s)
    End Sub

    Public Shared Sub M2(ByVal s As String) 
        Console.WriteLine("Shared method M2 on C:  s = {0}", s)
    End Sub
End Class

Public Class Example

    Public Shared Sub Main() 

        Dim c1 As New C(42)

        ' Get a MethodInfo for each method.
        '
        Dim mi1 As MethodInfo = GetType(C).GetMethod("M1", _
            BindingFlags.Public Or BindingFlags.Instance)
        Dim mi2 As MethodInfo = GetType(C).GetMethod("M2", _
            BindingFlags.Public Or BindingFlags.Static)

        Dim d1 As D1
        Dim d2 As D2
        Dim d3 As D3


        Console.WriteLine(vbLf & "An instance method closed over C.")
        ' In this case, the delegate and the
        ' method must have the same list of argument types; use
        ' delegate type D2 with instance method M1.
        '
        Dim test As [Delegate] = _
            [Delegate].CreateDelegate(GetType(D2), c1, mi1, False)

        ' Because False was specified for throwOnBindFailure 
        ' in the call to CreateDelegate, the variable 'test'
        ' contains Nothing if the method fails to bind (for 
        ' example, if mi1 happened to represent a method of 
        ' some class other than C).
        '
        If test IsNot Nothing Then
            d2 = CType(test, D2)

            ' The same instance of C is used every time the
            ' delegate is invoked.
            d2("Hello, World!")
            d2("Hi, Mom!")
        End If


        Console.WriteLine(vbLf & "An open instance method.")
        ' In this case, the delegate has one more 
        ' argument than the instance method; this argument comes
        ' at the beginning, and represents the hidden instance
        ' argument of the instance method. Use delegate type D1
        ' with instance method M1.
        '
        d1 = CType([Delegate].CreateDelegate(GetType(D1), Nothing, mi1), D1)

        ' An instance of C must be passed in each time the 
        ' delegate is invoked.
        '
        d1(c1, "Hello, World!")
        d1(New C(5280), "Hi, Mom!")


        Console.WriteLine(vbLf & "An open Shared method.")
        ' In this case, the delegate and the method must 
        ' have the same list of argument types; use delegate type
        ' D2 with Shared method M2.
        '
        d2 = CType([Delegate].CreateDelegate(GetType(D2), Nothing, mi2), D2)

        ' No instances of C are involved, because this is a Shared
        ' method. 
        '
        d2("Hello, World!")
        d2("Hi, Mom!")


        Console.WriteLine(vbLf & "A Shared method closed over the first argument (String).")
        ' The delegate must omit the first argument of the method.
        ' A string is passed as the firstArgument parameter, and 
        ' the delegate is bound to this string. Use delegate type 
        ' D3 with Shared method M2. 
        '
        d3 = CType([Delegate].CreateDelegate(GetType(D3), "Hello, World!", mi2), D3)

        ' Each time the delegate is invoked, the same string is
        ' used.
        d3()

    End Sub
End Class

' This code example produces the following output:
'
'An instance method closed over C.
'Instance method M1 on C:  id = 42, s = Hello, World!
'Instance method M1 on C:  id = 42, s = Hi, Mom!
'
'An open instance method.
'Instance method M1 on C:  id = 42, s = Hello, World!
'Instance method M1 on C:  id = 5280, s = Hi, Mom!
'
'An open Shared method.
'Shared method M2 on C:  s = Hello, World!
'Shared method M2 on C:  s = Hi, Mom!
'
'A Shared method closed over the first argument (String).
'Shared method M2 on C:  s = Hello, World!
' 

Example 2

The following code example demonstrates compatibility of parameter types and return types.

System_CAPS_noteNote

This code example uses the CreateDelegate(Type, MethodInfo) method overload. The use of other overloads that take MethodInfo is similar.

The code example defines a base class named Base and a class named Derived that derives from Base. The derived class has a static (Shared in Visual Basic) method named MyMethod with one parameter of type Base and a return type of Derived. The code example also defines a delegate named Example that has one parameter of type Derived and a return type of Base.

The code example demonstrates that the delegate named Example can be used to represent the method MyMethod. The method can be bound to the delegate because:

  • The parameter type of the delegate (Derived) is more restrictive than the parameter type of MyMethod (Base), so that it is always safe to pass the argument of the delegate to MyMethod.

  • The return type of MyMethod (Derived) is more restrictive than the parameter type of the delegate (Base), so that it is always safe to cast the return type of the method to the return type of the delegate.

The code example produces no output.

Imports System
Imports System.Reflection

' Define two classes to use in the demonstration, a base class and 
' a class that derives from it.
'
Public Class Base
End Class

Public Class Derived
    Inherits Base

    ' Define a Shared method to use in the demonstration. The method 
    ' takes an instance of Base and returns an instance of Derived.  
    ' For the purposes of the demonstration, it is not necessary for 
    ' the method to do anything useful. 
    '
    Public Shared Function MyMethod(ByVal arg As Base) As Derived
        Dim dummy As Base = arg
        Return New Derived()
    End Function

End Class

' Define a delegate that takes an instance of Derived and returns an
' instance of Base.
'
Public Delegate Function Example(ByVal arg As Derived) As Base

Module Test

    Sub Main()

        ' The binding flags needed to retrieve MyMethod.
        Dim flags As BindingFlags = _
            BindingFlags.Public Or BindingFlags.Static

        ' Get a MethodInfo that represents MyMethod.
        Dim minfo As MethodInfo = _
            GetType(Derived).GetMethod("MyMethod", flags)

        ' Demonstrate contravariance of parameter types and covariance
        ' of return types by using the delegate Example to represent
        ' MyMethod. The delegate binds to the method because the
        ' parameter of the delegate is more restrictive than the 
        ' parameter of the method (that is, the delegate accepts an
        ' instance of Derived, which can always be safely passed to
        ' a parameter of type Base), and the return type of MyMethod
        ' is more restrictive than the return type of Example (that
        ' is, the method returns an instance of Derived, which can
        ' always be safely cast to type Base). 
        '
        Dim ex As Example = CType( _
            [Delegate].CreateDelegate(GetType(Example), minfo), _
            Example _
        )

        ' Execute MyMethod using the delegate Example.
        '        
        Dim b As Base = ex(New Derived())
    End Sub
End Module

Example 3

The following code example shows all the methods a single delegate type can represent, using the CreateDelegate method to create the delegates.

System_CAPS_noteNote

There are two overloads of the CreateDelegate method that specify firstArgument and a MethodInfo; their functionality is the same except that one allows you to specify whether to throw on failure to bind, and the other always throws. This code example uses both overloads.

The code example defines two classes, C and F, and a delegate type D with one argument of type C. The classes have matching static and instance methods M1, M3, and M4, and class C also has an instance method M2 that has no arguments.

A third class named Example contains the code that creates the delegates.

  • Delegates are created for instance method M1 of type C and type F; each is closed over an instance of the respective type. Method M1 of type C displays the ID properties of the bound instance and of the argument.

  • A delegate is created for method M2 of type C. This is an open instance delegate, in which the argument of the delegate represents the hidden first argument on the instance method. The method has no other arguments. It is called as if it were a static method.

  • Delegates are created for static method M3 of type C and type F; these are open static delegates.

  • Finally, delegates are created for static method M4 of type C and type F; each method has the declaring type as its first argument, and an instance of the type is supplied, so the delegates are closed over their first arguments. Method M4 of type C displays the ID properties of the bound instance and of the argument.

Imports System
Imports System.Reflection
Imports System.Security.Permissions

' Declare a delegate type. The object of this code example
' is to show all the methods this delegate can bind to.
'
Public Delegate Sub D(ByVal c As C) 

' Declare two sample classes, C and F. Class C has an ID
' property so instances can be identified.
'
Public Class C

    Private _id As Integer

    Public ReadOnly Property ID() As Integer 
        Get
            Return _id
        End Get
    End Property

    Public Sub New(ByVal newId As Integer) 
        Me._id = newId
    End Sub

    Public Sub M1(ByVal c As C) 
        Console.WriteLine("Instance method M1(c As C) on C:  this.id = {0}, c.ID = {1}", _
            Me.id, c.ID)
    End Sub

    Public Sub M2() 
        Console.WriteLine("Instance method M2() on C:  this.id = {0}", Me.id)
    End Sub

    Public Shared Sub M3(ByVal c As C) 
        Console.WriteLine("Shared method M3(c As C) on C:  c.ID = {0}", c.ID)
    End Sub

    Public Shared Sub M4(ByVal c1 As C, ByVal c2 As C) 
        Console.WriteLine("Shared method M4(c1 As C, c2 As C) on C:  c1.ID = {0}, c2.ID = {1}", _
            c1.ID, c2.ID)
    End Sub
End Class


Public Class F

    Public Sub M1(ByVal c As C) 
        Console.WriteLine("Instance method M1(c As C) on F:  c.ID = {0}", c.ID)
    End Sub

    Public Shared Sub M3(ByVal c As C) 
        Console.WriteLine("Shared method M3(c As C) on F:  c.ID = {0}", c.ID)
    End Sub

    Public Shared Sub M4(ByVal f As F, ByVal c As C) 
        Console.WriteLine("Shared method M4(f As F, c As C) on F:  c.ID = {0}", c.ID)
    End Sub
End Class

Public Class Example

    Public Shared Sub Main() 

        Dim c1 As New C(42)
        Dim c2 As New C(1491)
        Dim f1 As New F()

        Dim d As D

        ' Instance method with one argument of type C.
        Dim cmi1 As MethodInfo = GetType(C).GetMethod("M1")
        ' Instance method with no arguments.
        Dim cmi2 As MethodInfo = GetType(C).GetMethod("M2")
        ' Shared method with one argument of type C.
        Dim cmi3 As MethodInfo = GetType(C).GetMethod("M3")
        ' Shared method with two arguments of type C.
        Dim cmi4 As MethodInfo = GetType(C).GetMethod("M4")

        ' Instance method with one argument of type C.
        Dim fmi1 As MethodInfo = GetType(F).GetMethod("M1")
        ' Shared method with one argument of type C.
        Dim fmi3 As MethodInfo = GetType(F).GetMethod("M3")
        ' Shared method with an argument of type F and an 
        ' argument of type C.
        Dim fmi4 As MethodInfo = GetType(F).GetMethod("M4")

        Console.WriteLine(vbLf & "An instance method on any type, with an argument of type C.")
        ' D can represent any instance method that exactly matches its
        ' signature. Methods on C and F are shown here.
        '
        d = CType([Delegate].CreateDelegate(GetType(D), c1, cmi1), D)
        d(c2)
        d = CType([Delegate].CreateDelegate(GetType(D), f1, fmi1), D)
        d(c2)

        Console.WriteLine(vbLf & "An instance method on C with no arguments.")
        ' D can represent an instance method on C that has no arguments;
        ' in this case, the argument of D represents the hidden first
        ' argument of any instance method. The delegate acts like a 
        ' Shared method, and an instance of C must be passed each time
        ' it is invoked.
        '
        d = CType([Delegate].CreateDelegate(GetType(D), Nothing, cmi2), D)
        d(c1)

        Console.WriteLine(vbLf & "A Shared method on any type, with an argument of type C.")
        ' D can represent any Shared method with the same signature.
        ' Methods on F and C are shown here.
        '
        d = CType([Delegate].CreateDelegate(GetType(D), Nothing, cmi3), D)
        d(c1)
        d = CType([Delegate].CreateDelegate(GetType(D), Nothing, fmi3), D)
        d(c1)

        Console.WriteLine(vbLf & "A Shared method on any type, with an argument of")
        Console.WriteLine("    that type and an argument of type C.")
        ' D can represent any Shared method with one argument of the
        ' type the method belongs and a second argument of type C.
        ' In this case, the method is closed over the instance of
        ' supplied for the its first argument, and acts like an instance
        ' method. Methods on F and C are shown here.
        '
        d = CType([Delegate].CreateDelegate(GetType(D), c1, cmi4), D)
        d(c2)
        Dim test As [Delegate] = _
            [Delegate].CreateDelegate(GetType(D), f1, fmi4, false)

        ' This final example specifies False for throwOnBindFailure 
        ' in the call to CreateDelegate, so the variable 'test'
        ' contains Nothing if the method fails to bind (for 
        ' example, if fmi4 happened to represent a method of  
        ' some class other than F).
        '
        If test IsNot Nothing Then
            d = CType(test, D)
            d(c2)
        End If

    End Sub
End Class

' This code example produces the following output:
'
'An instance method on any type, with an argument of type C.
'Instance method M1(c As C) on C:  this.id = 42, c.ID = 1491
'Instance method M1(c As C) on F:  c.ID = 1491
'
'An instance method on C with no arguments.
'Instance method M2() on C:  this.id = 42
'
'A Shared method on any type, with an argument of type C.
'Shared method M3(c As C) on C:  c.ID = 42
'Shared method M3(c As C) on F:  c.ID = 42
'
'A Shared method on any type, with an argument of
'    that type and an argument of type C.
'Shared method M4(c1 As C, c2 As C) on C:  c1.ID = 42, c2.ID = 1491
'Shared method M4(f As F, c As C) on F:  c.ID = 1491
'

ReflectionPermission

for accessing a non-public method when the grant set of the non-public method is restricted to the caller's grant set, or a subset thereof. Associated enumeration: ReflectionPermissionFlag.RestrictedMemberAccess

ReflectionPermission

for accessing a non-public method regardless of its grant set. Associated enumeration: ReflectionPermissionFlag.MemberAccess

ReflectionPermission

when invoked late-bound through mechanisms such as Type.InvokeMember. Associated enumeration: ReflectionPermissionFlag.MemberAccess.

.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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