Check if object is null without using ==

In my program, I have objects derived from Dictionary. I need to check if 2 objects are equal, so I made an overload operator ==.

But at a later point, I need to check if a object is null.

If (object == null)  
{...}

So at this point, the program goes into the overload operation I have defined, but will throw a NullReferenceException, since one of the objects to compare is null.

So within the overload operation, I need to check if one object is null, but without using ==, since that would give me a StackOverflowException.

How can I check this?

Solution

In my program, I have objects derived from Dictionary.

I would strongly reconsider deriving from Dictionary in the first place. That's very rarely a good idea. Favour composition over inheritance in general, and think very carefully about how you want equality (and hash codes) to behave in the face of mutable data. It's usually a bad sign if two objects are equal at one point and then not equal later.

So at this point, the program goes into the overload operation I have defined, but will throw a NullReferenceException, since one of the objects to compare is null.

That's the problem. Your overloaded == operator should not throw an exception - it should compare the value with null.

Typically an overload of == looks something like this:

public static bool ==(Foo left, Foo right)
{
    if (ReferenceEquals(left, right))
    {
        return true;
    }
    if (ReferenceEquals(left, null))
    {
        return false;
    }
    // Equals should be overridden, and should be handling the case where
    // right is null (by checking with ReferenceEquals too). Also consider
    // implementing IEquatable<Foo>
    return left.Equals(right);
}

Here Object.ReferenceEquals is used to perform the reference identity comparison. You could use this outside (in the code that you're currently considering changing) but it would be cleaner just to make == behave correctly.