memory leaks - Is this object-lifetime-extending-closure a C# compiler bug?

Question

I was answering a question about the possibility of closures (legitimately) extending object-lifetimes when I ran into some extremely curious code-gen on the part of the C# compiler (4.0 if that matters).

The shortest repro I can find is the following:

1. Create a lambda that captures a local while calling a static method of the containing type.
2. Assign the generated delegate-reference to an instance field of the containing object.

Result: The compiler creates a closure-object that references the object that created the lambda, when it has no reason to - the 'inner' target of the delegate is a static method, and the lambda-creating-object's instance members needn't be (and aren't) touched when the delegate is executed. Effectively, the compiler is acting like the programmer has captured this without reason.

class Foo
{
    private Action _field;

    public void InstanceMethod()
    {
        var capturedVariable = Math.Pow(42, 1);

        _field = () => StaticMethod(capturedVariable);
    }

    private static void StaticMethod(double arg) { }
}

The generated code from a release build (decompiled to 'simpler' C#) looks like this:

public void InstanceMethod()
{

    <>c__DisplayClass1 CS$<>8__locals2 = new <>c__DisplayClass1();

    CS$<>8__locals2.<>4__this = this; // What's this doing here?

    CS$<>8__locals2.capturedVariable = Math.Pow(42.0, 1.0);
    this._field = new Action(CS$<>8__locals2.<InstanceMethod>b__0);
}

[CompilerGenerated]
private sealed class <>c__DisplayClass1
{
    // Fields
    public Foo <>4__this; // Never read, only written to.
    public double capturedVariable;

    // Methods
    public void <InstanceMethod>b__0()
    {
        Foo.StaticMethod(this.capturedVariable);
    }
}

Observe that <>4__this field of the closure object is populated with an object reference but is never read from (there is no reason).

So what's going on here? Does the language-specification allow for it? Is this a compiler bug / oddity or is there a good reason (that I'm clearly missing) for the closure to reference the object? This makes me anxious because this looks like a recipe for closure-happy programmers (like me) to unwittingly introduce strange memory-leaks (imagine if the delegate were used as an event-handler) into programs.

Answer 1

That sure looks like a bug. Thanks for bringing it to my attention. I'll look into it. It is possible that it has already been found and fixed.