Why is the "this" pointer null in a delegate?

Question

I have the following code (details removed for clarity):

    private abstract class Base<TResult> {
        private readonly System.Func<TResult> func = null;

        protected Base(System.Func<TResult> func) {
            this.func = func;
        }

        public TResult Execute() {
            return this.func();
        }
    }

    private class Derived : Base<bool> {
        public Derived(bool myValue) : base(delegate() { return this.MyValue(); }) {
            this.myValue = myValue;
        }

        private bool myValue = false;
        private bool MyValue() {
            return this.myValue; // The "this" pointer is null here...
        }
    }

    Derived d = new Derived(true);
    bool result = d.Execute(); // This results in a null reference pointer (commented above)

Any ideas?

Thanks, Dave

Answer 1

Is that even legal? this is not defined at that point. IIRC, this is a compiler bug - already fixed in 4.0.

Here it is in the 4.0 compiler:

Error 1 Keyword 'this' is not available in the current context C:\Users\Marc\AppData\Local\Temporary Projects\ConsoleApplication1\Program.cs 22 40 ConsoleApplication1

To quote 7.5.7:

A this-access is permitted only in the block of an instance constructor, an instance method, or an instance accessor. It has one of the following meanings:

(emph mine)

...

Use of this in a primary-expression in a context other than the ones listed above is a compile-time error. In particular, it is not possible to refer to this in a static method, a static property accessor, or in a variable-initializer of a field declaration.

In the example given, it it simply invalid.

Answer 2

Using this in a constructor is always dangerous (except in the special case where you are invoking a sibling constructor). Your Derived constructor captures this at the time of its invocation, which is null as the instance hasn't been constructed yet.