(this == null) in C#!

Question

Due to a bug that was fixed in C# 4, the following program prints true. (Try it in LINQPad)

void Main() { new Derived(); }

class Base {
    pu         


        

Answer 1

The raw decompilation (Reflector with no optimizations) of the Debug mode binary is:

private class Derived : Program.Base
{
    // Methods
    public Derived()
    {
        base..ctor(new Func(Program.Derived.<.ctor>b__0));
        return;
    }

    [CompilerGenerated]
    private static string <.ctor>b__0()
    {
        string CS$1$0000;
        CS$1$0000 = CS$1$0000.CheckNull();
    Label_0009:
        return CS$1$0000;
    }

    private string CheckNull()
    {
        string CS$1$0000;
        CS$1$0000 = "Am I null? " + ((bool) (this == null));
    Label_0017:
        return CS$1$0000;
    }
}

The CompilerGenerated method doesn't make sense; if you look at the IL (below), it's calling the method on a null string (!).

   .locals init (
        [0] string CS$1$0000)
    L_0000: ldloc.0 
    L_0001: call instance string CompilerBug.Program/Derived::CheckNull()
    L_0006: stloc.0 
    L_0007: br.s L_0009
    L_0009: ldloc.0 
    L_000a: ret 

In Release mode, the local variable is optimized away, so it tries to push a non-existant variable on to the stack.

    L_0000: ldloc.0 
    L_0001: call instance string CompilerBug.Program/Derived::CheckNull()
    L_0006: ret 

(Reflector crashes when turning it into C#)

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EDIT: Does anyone (Eric Lippert?) know why the compiler emits the ldloc?