A delegate is an object that holds a reference to a method with a particular parameter list and return type.
It is basically like an object-oriented, type-safe function pointer.
Delegates basically make it possible to treat methods as entities. You can then assign a delegate to a variable or pass it as a parameter.
The classic delegate example uses events.
In C#:
HelloButton.Click += new EventHandler(HelloButton_Click);
Or the short-cut version of this code:
HelloButton.Click += HelloButton_Click;
This passes the HelloButton_Click method to a new EventHandler delegate.
In VB:
AddHandler HelloButton.Click, AddressOf HelloButton_Click
In VB, the AddressOf assigns the address of the HelloButton_Click method to the delegate.
Instead of using a named method for the delegate (in this example, a HelloButton_Click method), you can use a lambda expression.
In C#:
HelloButton.Click += (s, ev) =>
MessageBox.Show("Hello World");
In VB:
AddHandler HelloButton.Click, Function(s, ev) _
MessageBox.Show("Hello World")
In this case the lambda expression has two parameters: s (sender) and ev (eventArgs). When the Click event is generated on the HelloButton, the code displays the MessageBox.
You can use a lambda expression (inline function) anywhere a delegate is required. However, if the function requires more than a few lines of code, a named method is the preferred and recommended technique.
Enjoy!