Using extension methods from within the class they extend

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();
}

Extension methods don't actually extend a type. They only appear to thanks to IDE and compiler tricks. — Oded, Sep 10 '13 at 15:39
btw, I can easily type `this`, I'm just OCD about coding neatness, and the keyword annoys me!!! — BG100, Sep 10 '13 at 15:40
Damn... I did try to search for dup questions first... but couldn't find anything!! obviously didn't look hard enough. — BG100, Sep 10 '13 at 15:46
It can also be called like a normal static method `ThingExtensions.TestForBlue(this)`. — Olivier Jacot-Descombes, Sep 10 '13 at 15:49

score 14 · Accepted Answer · edited Jun 20 '20 at 09:12

14

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.

edited Jun 20 '20 at 09:12

Community

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1

answered Sep 10 '13 at 15:41

Jon Skeet

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From which document does Section 7.6.5.2 refer? Google doesn't seem to know... – BG100 Sep 10 '13 at 15:44
1

@BG100 The C# 5.0 language specifications. – Servy Sep 10 '13 at 15:47
@BG100: The specification is free at http://www.microsoft.com/en-us/download/details.aspx?id=7029 . – Brian Sep 10 '13 at 21:10

score 0 · Answer 2 · answered Sep 10 '13 at 15:41

0

Because you need to invoke which extension method type is it gonna call, since the Extension is defined as Thing, the object needs to call itself and the static method that was defined for it.

answered Sep 10 '13 at 15:41

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The C# specs could have easily checked the calling class (and base classes) and checked for us. They just didn't. I can only assume at some point they will add this in for us. – user441521 Aug 17 '16 at 21:17

Using extension methods from within the class they extend

2 Answers2