What does the this keyword in a method declaration mean?

Question

I found a method declared like this:

public static int WordCount(this string str)
{
    return str.Length;
}

What's the this keyword in this particular context?

@BrianRasmussen [It is](http://stackoverflow.com/questions/846766/use-of-this-keyword-in-formal-parameters-for-static-methods-in-c-sharp). — It'sNotALie., Aug 19 '13 at 18:21
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score 2 · Accepted Answer · answered Aug 19 '13 at 18:17

It's an extension method. Extension methods allow you to extend a type. It's great for types like string for which you don't have access to the source code.

With the example you provided, instead of calling

string test = "foo";
var result = StaticClass.WordCount(test);

You could use it as follows:

string test = "foo";
var result = test.WordCount();

Trivia: LINQ is implemented using extension methods, and in fact was the primary reason extension methods were added to the .NET framework.

score 0 · Answer 2 · answered Aug 19 '13 at 18:17

It makes it an extension method.

What this means is that you can call the method like this:

arg.WordCount();

instead of like this:

Class.WordCount(arg);

It's mainly used for extending types (hence the name) that you can't directly modify the source code of.

What does the this keyword in a method declaration mean?

2 Answers2