Extension methods not available on base types?

Question

[ Likely a duplicate but I've not found it ]

I was surprised to receive the error 'List<int>' does not contain a definition for 'Sum' during the below usage?

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;

namespace Test
{
    public class Foo: List<int>
    {
        public int Total => base.Sum();
    }
}

It seems that extension methods are not available after inheritance? Or I'm missing something?

P.S. I'm aware of Why not inherit from List?

You'll need to use the `this` keyword to access the linq extension methods: `Total => this.Sum()` — Metro Smurf, Aug 21 '22 at 18:34
`base` is used to access members of the base class, not extension methods. — DavidG, Aug 21 '22 at 18:36
Why are you subclassing `List`? I really can't think of any good reason to do that. — Dai, Aug 21 '22 at 18:39

score 3 · Accepted Answer · answered Aug 21 '22 at 18:40

The key to understanding this behavior is that, contrary to this, base itself is not an expression. C# specification for the base keyword (11.7.13 Base access):

Note: Unlike this, base is not an expression in itself. It is a keyword only used in the context of a base_access or a constructor_initializer (§14.11.2). end note

If you look at the C# specification for extension methods (11.7.8.3 Extension method invocations) you'll notice that extension methods apply only to expressions:

In a method invocation (§11.6.6.2) 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. If «expr» or any of the «args» has compile-time type dynamic, extension methods will not apply.

For the same reason of base itself not being an expression, you can't do these things with base either:

var stuff = base;
CallSomeMethod(base);

score 0 · Answer 2 · answered Aug 21 '22 at 18:34

base is there to be able to call the original method when you have redefined it, so it's looking for an implementation explicitely on the base type. Extension methods are not part of that type, they still work just fine you just don't need the base keyword here

public int Total => this.Sum();

Works just fine

Extension methods not available on base types?

2 Answers2

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