Delegate Statement
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 types and return type may be used to create an instance of this delegate class. The procedure can then later be invoked by means of the delegate instance.
[ <attrlist> ] [ accessmodifier ] _
[ Shadows ] Delegate [ Sub | Function ] name [( Of typeparamlist )] [([ parameterlist ])] [ As type ]
Parts
- 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.
You can specify Protected Friend to allow access from code within the delegate's class, a derived class, or the same assembly.
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. 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 types and return type of a delegate class. Any procedure with matching parameters types and return type may 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 an expression of the form:
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.
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