Using extension methods from within the class they extend [duplicate]

Question

Consider this class:

public class Thing {

    public string Color { get; set; }

    public bool IsBlue() {
        return this.Color == "Blue";   // redundant "this"
    }

}

I can omit the keyword this because Color is a property of Thing, and I'm coding within Thing.

If I now create an extension method:

public static class ThingExtensions {

    public static bool TestForBlue(this Thing t) {
        return t.Color == "Blue";
    }

}

I can now change my IsBlue method to this:

public class Thing {

    public string Color { get; set; }

    public bool IsBlue() {
        return this.TestForBlue();   // "this" is now required
    }
}

However, I'm now required to include the this keyword.

I can omit this when referencing properties and methods, so why can't I do this...?

public bool IsBlue() {
    return TestForBlue();
}

Answer 1

I can omit this when referencing properties and methods, so why can't I do this...?

It's just part of how extension methods are invoked, basically. Section 7.6.5.2 of the C# specification (Extension Method Invocations) starts:

In a method invocation (7.5.5.1) of one of the forms

expr . identifier ( )
expr . identifier ( args )
expr . identifier < typeargs > ( )
expr . identifier < typeargs > ( args )

if the normal processing of the invocation finds no applicable methods, an attempt is made to process the construct as an extension method invocation.

Without the this, your invocation wouldn't be of that form, so that section of the spec wouldn't apply.

This isn't a justification of why the feature was designed that way, of course - it's a justification of the compiler's behaviour in terms of correctness.