Extension methods allow developers to add new methods to the public
contract of an existing CLR type,
without having to sub-class it or
recompile the original type.Extension Methods help blend the
flexibility of “duck typing” support
popular within dynamic languages today
with the performance and compile-time
validation of strongly-typed
languages.
Reference: http://weblogs.asp.net/scottgu/archive/2007/03/13/new-orcas-language-feature-extension-methods.aspx
Here is a sample of an Extension Method (notice the this keyword infront of the first parameter):
public static bool IsValidEmailAddress(this string s)
{
Regex regex = new Regex(@"^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$");
return regex.IsMatch(s);
}
Now, the above method can be called directly from any string, like such:
bool isValid = "[email protected]".IsValidEmailAddress();
The added methods will then also appear in IntelliSense:
(source: scottgu.com)
As regards a practical use for Extension Methods, you might add new methods to a class without deriving a new class.
Take a look at the following example:
public class Extended {
public int Sum() {
return 7+3+2;
}
}
public static class Extending {
public static float Average(this Extended extnd) {
return extnd.Sum() / 3;
}
}
As you see, the class Extending is adding a method named average to class Extended. To get the average, you call average method, as it belongs to extended class:
Extended ex = new Extended();
Console.WriteLine(ex.average());