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Delegate Statement

  • Άρθρο

Used to declare a delegate. A delegate is a reference type that refers to a Shared method of a type or to an instance method of an object. Any procedure with matching parameter and return types can be used to create an instance of this delegate class. The procedure can then later be invoked by means of the delegate instance.

Syntax

[ <attrlist> ] [ accessmodifier ] _  
[ Shadows ] Delegate [ Sub | Function ] name [( Of typeparamlist )] [([ parameterlist ])] [ As type ]

Parts

Term Definition
attrlist Optional. List of attributes that apply to this delegate. Multiple attributes are separated by commas. You must enclose the Attribute List in angle brackets ("<" and ">").
accessmodifier Optional. Specifies what code can access the delegate. Can be one of the following:

- Public. Any code that can access the element that declares the delegate can access it.
- Protected. Only code within the delegate's class or a derived class can access it.
- Friend. Only code within the same assembly can access the delegate.
- Private. Only code within the element that declares the delegate can access it.

- Protected Friend Only code within the delegate's class, a derived class, or the same assembly can access the delegate.
- Private Protected Only code within the delegate's class or in a derived class in the same assembly can access the delegate.
Shadows Optional. Indicates that this delegate redeclares and hides an identically named programming element, or set of overloaded elements, in a base class. You can shadow any kind of declared element with any other kind.

A shadowed element is unavailable from within the derived class that shadows it, except from where the shadowing element is inaccessible. For example, if a Private element shadows a base class element, code that does not have permission to access the Private element accesses the base class element instead.
Sub Optional, but either Sub or Function must appear. Declares this procedure as a delegate Sub procedure that does not return a value.
Function Optional, but either Sub or Function must appear. Declares this procedure as a delegate Function procedure that returns a value.
name Required. Name of the delegate type; follows standard variable naming conventions.
typeparamlist Optional. List of type parameters for this delegate. Multiple type parameters are separated by commas. Optionally, each type parameter can be declared variant by using In and Out generic modifiers. You must enclose the Type List in parentheses and introduce it with the Of keyword.
parameterlist Optional. List of parameters that are passed to the procedure when it is called. You must enclose the Parameter List in parentheses.
type Required if you specify a Function procedure. Data type of the return value.

Remarks

The Delegate statement defines the parameter and return types of a delegate class. Any procedure with matching parameters and return types can be used to create an instance of this delegate class. The procedure can then later be invoked by means of the delegate instance, by calling the delegate's Invoke method.

Delegates can be declared at the namespace, module, class, or structure level, but not within a procedure.

Each delegate class defines a constructor that is passed the specification of an object method. An argument to a delegate constructor must be a reference to a method, or a lambda expression.

To specify a reference to a method, use the following syntax:

AddressOf [expression.]methodname

The compile-time type of the expression must be the name of a class or an interface that contains a method of the specified name whose signature matches the signature of the delegate class. The methodname can be either a shared method or an instance method. The methodname is not optional, even if you create a delegate for the default method of the class.

To specify a lambda expression, use the following syntax:

Function ([parm As type, parm2 As type2, ...]) expression

The signature of the function must match that of the delegate type. For more information about lambda expressions, see Lambda Expressions.

For more information about delegates, see Delegates.

Example

The following example uses the Delegate statement to declare a delegate for operating on two numbers and returning a number. The DelegateTest method takes an instance of a delegate of this type and uses it to operate on pairs of numbers.

Delegate Function MathOperator( 
    ByVal x As Double, 
    ByVal y As Double 
) As Double

Function AddNumbers( 
    ByVal x As Double, 
    ByVal y As Double 
) As Double
    Return x + y
End Function

Function SubtractNumbers( 
    ByVal x As Double, 
    ByVal y As Double
) As Double
    Return x - y
End Function

Sub DelegateTest( 
    ByVal x As Double, 
    ByVal op As MathOperator, 
    ByVal y As Double 
)
    Dim ret As Double
    ret = op.Invoke(x, y) ' Call the method.
    MsgBox(ret)
End Sub

Protected Sub Test()
    DelegateTest(5, AddressOf AddNumbers, 3)
    DelegateTest(9, AddressOf SubtractNumbers, 3)
End Sub

See also