What is an extension method?

ExtensionMethods are a nice feature added in C# 3.0 that allow us to 'pretend' that a method is an instance method of an object where really it's defined in a static class outside of the type. This small difference can make a huge step forward in readability:

Code with Linq query like this:

var query2 = people
			.Where(p => p.Age > 20)
			.OrderByDescending(p => p.Age)
			.ThenBy(p => p.Name));

will look like this without extension methods:

var query2 = Enumerable.ThenBy(
                Enumerable.OrderBy(
                   Enumerable.Where(
                        people, 
                   p => p.Age > 20),
                p => p.Age),
             p => p.Name):

because all these methods are not implemented over IEnumerable<T>.

This language feature has become so useful that many utility projects have grown around the concept, an active question about this.

It looks like everybody wants to make an 'standard' set of extension methods, and in the way they introduce a new library, making the problem bigger.

So, why bother doing a new one?

• Consistency: It looks better for Signum Framework to depend on Signum.Utilities than on an external library.
• Control: We prefer to have control over the library so we can add code we need.
• Clutter: Extension methods tend to create clutter on your IntelliSense. We follow Framework Design Guidelines.