Question

Why does the C# compiler not even complain with a warning on this code? :

if (this == null)
{
   // ...
}

Obviously the condition will never be satisfied..

Answer 1

Because you could override operator == to return true for that case.

public class Foo
{
    public void Test()
    {
        Console.WriteLine(this == null);
    }

    public static bool operator ==(Foo a, Foo b)
    {
        return true;
    }

    public static bool operator !=(Foo a, Foo b)
    {
        return true;
    }
}

Running new Foo().Test() will print “True” to the console.

The other question here is: why doesn’t the compiler issue a warning for ReferenceEquals(this, null)? From the bottom of the above link:

A common error in overloads of operator == is to use (a == b), (a == null), or (b == null) to check for reference equality. This instead results in a call to the overloaded operator ==, causing an infinite loop. Use ReferenceEquals or cast the type to Object, to avoid the loop.

That might be answered by @Aaronaught’s response. And that’s also why you should be doing (object)x == null or ReferenceEquals(x, null), not doing a simple x == null, when you’re checking for null references. Unless, of course, you’re sure that the == operator is not overloaded.