Delegate:
Delegates play an important roll in the
programming model of the .NET framework. They provide valuable support for
developers who need to design and write software that use the callback
notification. for example Event-handling is based on Callback.
Callback:
Application software often designed around the programming technique known
as Callback. In a callback, one part of an application sends out notification to
alert other parts of an application when something interesting has occurred.
More specifically, a callback is call from a notification source back to method
implemented by one or more handlers.
Definition of Delegates:
Delegates is nothing but, it is work as a function pointer.
Define Delegates:
In Visual Basic .NET, you define a delegate type by using the Delegate
key word each delegate definition you create must include a type name and
calling signature for handler method.
Public Delegate Sub
delegatename1()
'Deligate without parameters
Public Delegate Sub
delegatename2(ByVal a
As Integer)
'Deligate with parameters
Note: There are various points to remember during delegates declaration
- Delegate can not use as a Private Delegate.
- Out parameter is not allowed in Delegate.
- Delegate type must be defined using either the sub keyword or the function keyword.
Creating Delegate Object:-When you want
to create a delegate object, you typically use
the new operator followed by the name of the delegate type.
Dim
Handler1 As delegatename1
Handler1=new delegatename1(Addressof
objectname.methodname)
Note: There are various points to
remember during delegates object creation
- Handler1 is not an object, it is a reference Id variable that can hold all reference id.
- Before you can create delegate object, you must determine the handler method to
which it should bind. A handler method can be either a shared or an instance method.
Coding for Delegates:
Module Module1
Public Delegate Sub Display()
Public Class
Par
Public Sub
View1()
Console.WriteLine("hello world")
End Sub
End Class
Class Chal :
Inherits Par
Public Sub
View2()
Console.WriteLine("World is Beautiful")
End Sub
End Class
Sub Main()
Dim obj As New Chal
Dim obj1 As
Display
Dim obj2 As
Display
obj1 = New Display(AddressOf
obj.View1)
obj2 = New Display(AddressOf
obj.View2)
obj1.Invoke()
obj2.Invoke()
End Sub
End Module
Benefits of Delegates:-There
are various Advantages of delegates
-
Delegates provide one of the easiest ways to become involved in multithreading.
-
Delegates are useful to use the event handling.
-
delegates are also useful to window forms and asp.net.
Output:
