User FAQs

1) What is a Delegate ?
A delegate represents a reference to a method that has a particular signature (parameter list and return type).
Delegates give you a way of calling a method at run-time.
When you instantiate a delegate you can associate its instance with any method with a matching signature.
Delegates are used to pass methods as arguments to other methods.

public delegate int MyDelegate(int x); 

Examples include: type safe functions, pointers, callbacks.

2) Delegates with Named Methods
When you instantiate a delegate by using a named method, the method is passed as the parameter.

delegate int MyDelegate(int x); 
void DoSomething(int a) 
{ return a * a; } 
MyDelegate del = DoSomething; 

3) Delegates with Anonymous Methods
Introduced in C# 2.0 (.NET 2.0).
An anonymous method provides a way of passing a block of code as an argument instead of creating an explicit named method.

delegate int MyDelegate (int x); 
MyDelegate del = delegate (int a) 
   { return a * a; } 

4) What is a Lambda Expression ?
Introduced in C# 3.0 (.NET 3.5).
A lambda expression is a more concise way of writing an anonymous function.
You specify the input parameters (if any) on the left of the lambda operator => and the statement block (or expression) on the right.

delegate int MyDelegate (int x); 
MyDelegate del = a => a * a; 

These are often used to create delegates for use in LINQ queries.

5) What is the difference between a Delegate and an Event ?
An event is something that can be added or removed from an object.
When you add or create an event you are storing a delegate reference which can then be called at run-time.

6) What is an Event ?
An event is an indication that a state is about to change or that a state has changed.
An event handler is a method that is invoked using a delegate.

button.click += delegate(System.Object o, _ 
                         System.EventArgs e) 
                        { MessageBox.Show("pressed"); }; 

7) What is a Multicast Delegate ?
A Delegate object holds a reference to a single method.
It is possible for a delegate object to hold references to multiple methods.
These delegates are called multicast or combinable delegates.

8) What is a Generic Delegate ?
There are three different types of generic delegate.
System.Action - can include 0 to 16 input parameters, no return type
??
System.Func - can include 0 to 16 input parameters and one return type

System.Func<string, string> MyDelegate = MyMethod; 
public static string MyMethod(string inputstring) 
{ return ""; } 

System.Predicate - can verify a certain criteria and return a boolean value

9) Can you describe some situations when you would use Delegates ?
Callback Mechanism -
Asynchronous Processing -
Multi-Casting -
Abstract and Encapsulate Methods -