Can extension methods access the "this" object?

Question

Lets say I want to add an extension method to class B. Can I get a reference to the instance of class B the extension method is invoked on by using the "this" reference inside my extension method?

score 6 · Accepted Answer · answered Apr 06 '11 at 05:25

Yes and no. A short look at the documentation makes it VERY clear.

Per definition the first parameter of an extension method is the pointer to the object the method was called from / attached to, and it actually is a variable referenced by the this keyword but with it's own name:

http://msdn.microsoft.com/en-us/library/bb383977.aspx

namespace ExtensionMethods
{
    public static class MyExtensions
    {
        public static int WordCount(this String str)
        {
            return str.Split(new char[] { ' ', '.', '?' }, 
                             StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }   
}

That makes it quite easy that there is a "this" there, named "str". So, you can not use "this" (because that would point to the non-existing instance of the class the extension method is defined on), but you can declare your own replacement variable pointing to the object an extension method is attached to.

score 1 · Answer 2 · answered Apr 06 '11 at 05:22

1

No; you have to use the actual name of the argument.

answered Apr 06 '11 at 05:22

user541686

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Name of the *parameter* rather than argument, but otherwise yes. – Jon Skeet Apr 06 '11 at 05:26
@Jon Skeet: [Or just *parameter*?](http://stackoverflow.com/questions/1663705/difference-between-arguments-parameters-in-c/1663724#1663724) – BoltClock Apr 06 '11 at 05:30
@Jon: Way to be picky :P but yes, "argument" is not the correct term. I stand corrected. – user541686 Apr 06 '11 at 05:33

Can extension methods access the "this" object?

2 Answers2