Quick Tip: Checking for a reference type null in C#

How do you test for a null reference type in C#?

You'd think that it'd be easy to check for a null, just use some code like this:

    MyType thing = null;
    ...
    if ( thing == null)
    {
        // do something
    }

But what if you need to test for a null within an overloaded == operator?! You can't use the simple code above, as this would cause a stack overflow.

The trick is in casting the entity to a different type and so that the compiler uses the == for that alternate type. I wrap this test into a static IsNull() method on the class.

    public static bool IsNull(MyType thing)
    {
        object testThing = thing as object;

        // uses the == operator from object, NOT MyType
        return (testThing == null);
    }

So now I can safely test for equality, including correctly testing for nulls like this

    public static bool operator == (MyType thing1, MyType thing2)
    {
        // two nulls => equal
        if (IsNull(thing1) && IsNull(thing2))
            return true;

        // one null => not equal
        if (IsNull(thing1) || IsNull(thing2))
            return false;

        // do the REAL equality test here
        ...
     }

Simple when you know how! And absolutely essential when you're implementing IEquatable.