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C# Programming Guide

Delegates (C# Programming Guide)

A delegate is a type that references a method. Once a delegate is assigned a method, it behaves exactly like that method. The delegate method can be used like any other method, with parameters and a return value, as in this example:

public delegate int PerformCalculation(int x, int y);

Any method that matches the delegate's signature, which consists of the return type and parameters, can be assigned to the delegate. This makes is possible to programmatically change method calls, and also plug new code into existing classes. As long as you know the delegate's signature, you can assign your own delegated method.

This ability to refer to a method as a parameter makes delegates ideal for defining callback methods. For example, a sort algorithm could be passed a reference to the method that compares two objects. Separating the comparison code allows the algorithm to be written in a more general way.

Delegates Overview

Delegates have the following properties:

Delegates are similar to C++ function pointers, but are type safe.
Delegates allow methods to be passed as parameters.
Delegates can be used to define callback methods.
Delegates can be chained together; for example, multiple methods can be called on a single event.
Methods don't need to match the delegate signature exactly. For more information, see Covariance and Contravariance
C# version 2.0 introduces the concept of Anonymous Methods, which permit code blocks to be passed as parameters in place of a separately defined method.

In This Section

C# Language Specification

For more information, see the following sections in the C# Language Specification:

1.11 Delegates
4.2.6 Delegate types
7.5.5.2 Delegate invocations
15 Delegates

"A delegate is a type that references a method. Once a delegate is assigned a method, it behaves exactly like that method."

Does then "delegate" mean a type or an object?!

...It cannot be both. It seems to me that you have a single word for two meanings:
(1) a type containing a reference to a method of some specified signature,
(2) an object of that type, which can be actually called like a method.

I would prefer you to be more precise and use "delegate type" for the first case. I have been recently reading a lot about events and delegates and that ambiguity was making me confused many times.

Some other uses of "delegate" word in MSDN in the first meaning:
"Custom event delegates are needed only when an event generates event data"
"A delegate declaration defines a class that is derived from the class

System.Delegate"

Some other uses of "delegate" word in MSDN in the second meaning:
"specify a delegate that will be called upon the occurrence of some event"
"Delegates are objects that refer to methods. They are sometimes described as type-safe function pointers"

Delegates Overview

In This Section

C# Language Specification

See Also

Reference

Concepts