Extension methods are static methods that can be invoked using instance method syntax. In effect, extension methods make it possible to extend existing types and constructed types with additional methods. -- C# 3.0 Specification
Sounds good, so how can I make my method extension method?
A method became the extension method when the first method parameter contains `this’ modifier. Extension methods can be declared in non-generic top-level static classes which mean that extension methods can be declared as static only. There are further limitations but they only covering corner cases. For those interested, I recommend browsing Mono compiler test suite to get some ideas.
static class MyExtension
public static string Prefix (this string s, string prefix)
return prefix + s;
public static void Main ()
string s = "abc".Prefix ("1");
Note: If you want to use extension methods you need to pass `langversion:linq’ option to compiler to unlock C# 3.0 specific features; although extension methods are included in gmcs compiler.
When you try to compile the sample, it will compile successfully and an instance method call will be converted to a static method call by compiler. How is it possible? The method `Prefix’ is the extension method of `string’ class and no other method with same parameters exists and therefore the method is selected as the best overload candidate.
Wishing to use extension methods in more advanced scenario, things become little bit more complicated and you have to use `using
Confusing, isn’t it? Well, not that much. Extension methods are included in method resolving process if standard (non-extension) process failed to find a match. The compiler will look for extension methods in the current namespace context and if no suitable method was found, the process will continue inspecting all referenced namespaces in the current namespace. In the case of method was not found, the compiler continues in parent namespace until either method is found or parent namespace does not exist.
My overall feeling about the feature is mixed as extension methods clearly reduce code readability. Fortunately, I was pleased to read the recommendation "to use extension methods sparingly and only if it is not possible to use instance methods" directly inside C# Standard.