Visual Basic .NET - Delegates and Events (Advanced) in VB.NET

Delegates and events are important concepts in VB.NET that allow programs to respond to actions such as button clicks, mouse movements, or changes in data. In advanced programming, they are also used to build flexible and reusable applications.

1. What is a Delegate?

A delegate is a type that represents a reference to a method. It allows you to store the address of a method and call it later.

In simple words, a delegate is like a pointer to a function. It can hold a reference to any method that matches its signature (same return type and parameters).

Example:

Delegate Function Calculate(ByVal a As Integer, ByVal b As Integer) As Integer

Here, Calculate can point to any function that takes two integers and returns an integer.

Then you can assign a method to it:

Dim calc As Calculate = AddressOf Add

Now calc(5, 3) will call the Add method.

2. Why Delegates Are Important

Delegates are mainly used when:

  • You want to pass a method as a parameter to another method.

  • You want to execute different methods dynamically.

  • You are working with events.

They provide flexibility and support callback methods.

3. What is an Event?

An event is a way for a class to notify other classes when something happens.

For example:

  • A button click

  • A file download completion

  • A value change

Events are built using delegates. When an event occurs, the delegate calls the attached method automatically.

Example:

Public Class Sample
    Public Event ProcessCompleted()

    Public Sub StartProcess()
        ' Some work here
        RaiseEvent ProcessCompleted()
    End Sub
End Class

Another class can handle this event:

AddHandler obj.ProcessCompleted, AddressOf ShowMessage

When RaiseEvent is executed, ShowMessage will run.

4. Custom Events (Advanced Concept)

In advanced usage, you can create custom events with specific arguments using EventArgs.

Example:

Public Class MyEventArgs
    Inherits EventArgs
    Public Property Message As String
End Class

Then define the event:

Public Event DataSent As EventHandler(Of MyEventArgs)

This allows you to pass additional information when raising the event.

5. Multicast Delegates

A delegate can hold references to multiple methods at the same time. This is called a multicast delegate.

Example:

AddHandler obj.ProcessCompleted, AddressOf Method1
AddHandler obj.ProcessCompleted, AddressOf Method2

Now when the event is raised, both methods will execute.

6. Real-World Usage

Delegates and events are used in:

  • Windows Forms applications

  • WPF applications

  • Asynchronous programming

  • Callback mechanisms

  • Data binding

They help create loosely coupled systems where one class does not need to know details about another class.

Conclusion

Delegates allow methods to be treated like variables. Events use delegates to notify other parts of the program when something happens. Advanced understanding of delegates and events helps in building scalable, flexible, and event-driven applications in VB.NET.