Question

I did go through some MSDN documentations. I found extension methods also documented there.

I didn’t understand, why these base class libraries have extension methods? when they could have been added to that particular class library itself?

what is the advantage? difference ?

Answer 1

Extension methods as name suggests extends the functionality or usability of the target type.

1. These methods can be added later (than type authoring time) after type has already been published.
2. They can be written by different group of people
3. Extension methods can target interfaces. (Alternative would have been to have a common base type having these methods or re-implementing them in each type)
4. Different people can extend the same type differently as per their needs.

Correct use of extension methods can remove orthogonal clutter from the actual type definition/implementation (instead focusing of the core functionality of the type within type definition).

Take example of LINQ – by providing extension methods to IEnumerable, it could target vast number of already published types (and vast number of types that may written in future); it has separated orthogonal concern such as querying the type from the actual type.