Extension Methods

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Object-oriented Programming in C#
Data Access, Properties, and Methods

Extension Methods

Have you ever wanted to add a new method to an existing class, without defining a subclass? If so, extension methods - as introduced in C# 3.0 - is the solution to your problem.

In C# 3.0 an extension method defines an instance method in an existing class without altering the definition of the class.

Class Point and a client

Motivation: A Client of Point without use of DistanceTo.

A Client of class Point which uses an extension method DistanceTo.

The method DistanceTo is used as an instance method in this program...

The static class PointExtensions.

... but actually implemented as a (somewhat special) static method in the static class PointExtensions.

An extension method
- extends the type of the first parameter
- is defined as a static method with a this modifier on the first parameter
- must be defined in a static class
- cannot access private instance variables in the extended class
- is called by - behind the scene - translating to a call of the static method

Extending struct Double