Hi All,
What is the purpose of delegate Can't we call the methods directly instead of assigning to the AddressOf (refer my example). Please tell me the real trime implementation of delegate.
EX:
Private
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
Dim del As MyDelSub
del = New MyDelSub(AddressOf DisplayMessage)
del.Invoke()
End Sub Private Sub DisplayMessage()
MessageBox.Show("Delegate - Demo")
End Sub
Thanks & Regards,
Sriram S
How are Delegates used?
Mihir Vaidya
For example
Private Delegate Sub MyDelSub(ByVal sender as Int32)
ensures that any delegate that is passed of the type MyDelSub must accept an integer.
Alias2006
hi,
more over to what david said , the events use delegate extensively,
who wrote the button class that you use in your forms doesn't know which method will be used to handle this click event so he used delegate, you have to register with this event through delegate
i think of it as pattern for methods that will use this event
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
AddHandler Button1.Click, AddressOf mymethod
End Sub Private Sub mymethod(ByVal sender As Object, ByVal e As EventArgs)
MessageBox.Show("this button is clicked")
End Sub
hope this helps
Hong bo Dai
Lets not forget one of the most important functions for the humble delegate is in the use of Async data callbacks... without them, realtime data aware applications would not be possible and your screen would hang most of the time...
Cheers & Beers
Roland Hasenöhrl
Your example is basically a function pointer
Delegates can be described as a managed version of a function pointer.
A class that needs to raise events must define one delegate per event.
Types that use the class must implement one event handler method per event that must be processed.
The Difference between a delegate and function pointer is, delegates can reference both instance and static (also called shared) methods, while function pointers can only reference static methods.
Purpose of delegates
Its a mechanism used to implement event handling in .NET Framework code. Also a class that needs to raise events must define one delegate per event.
Mirronelli
Delegates allows you to call methods that aren't known at compile time. A good example of this is the List(Of T) type, it makes extensive use of delegates to perform certain functions, for example sorting:
Module Module1
Sub Main()
Dim list As New List(Of Integer)
list.Add(2)
list.Add(4)
list.Add(1)
list.Add(5)
list.Add(3)
For Each number As Integer In list
Console.WriteLine(number)
Next
list.Sort(AddressOf SortNumbers)
For Each number As Integer In list
Console.WriteLine(number)
Next
End Sub
Function SortNumbers(ByVal one As Integer, ByVal two As Integer) As Integer
Return one < two
End Function
End Module
In the above example, the developers of the List(Of T).Sort method do not know of the SortNumbers method at compile time, so instead the Sort method takes a delegate which gets called at runtime to determine if one number is greater than the other.