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Possible Duplicate:
extension method requires class to be static
In .NET:
Why can't static method in non-static class be an extension method?
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Just like static members, a static nested class does not have access to the instance variables and methods of the outer class. You can extend static inner class with another inner class.
Here, this keyword is used for binding, Geek is the class name in which you want to bind, and g is the parameter name. Extension methods are always defined as a static method, but when they are bound with any class or structure they will convert into non-static methods.
Extension methods are static because they get the instance passed in via the first parameter, and they don't act on the actual instance of their declaring class. Also, they're just a syntactic sugar. CLR doesn't support such a thing.
The only difference between a regular static method and an extension method is that the first parameter of the extension method specifies the type that it is going to operator on, preceded by the this keyword.
It makes sense that a static class is required to have extension methods. The number one reason is that the static class is stateless ...i.e. you don't have to instance the class. ...but this is just my gut feeling. It wouldn't make sense to me otherwise.
I think, too, that forcing extension methods to reside in public/internal static classes reduces the cost of using them.
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Eric Lippert will probably weigh in with a really good answer on this one, but the gist of it will probably be:
We decided it would be easier on both programmers and the compiler if we limit the number of places that you have to look for extension methods.
This policy tends to force users to put all of their extension methods into a few specific classes that are designated for this purpose.
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