The Johnson Blog

I recently came to a situation where operator overloading C# (specifically the == operator) would really be beneficial. Since I’ve typically steered away from this technique, for readability and maintainability, I hadn’t had much experience with it.

My overloaded operator looks something like this:


static bool operator==(Id<T> a, Id<T> b)
{
return a.Value == b.Value;
}


That has been working just as planned for a few months now, but today I hit a snag when trying to do a null test against a variable of that class type, like:


Id<int> x;
...
if (x == null)
{
...
}

This causes a nice null reference exception at a.Value because a is null. My first thought was to test for null in the operator== method before I attempt accessing a.Value (or b.Value), like:

if (a == null)
{
return b == null;
}
else
{
return a.Value == b.Value;
}

Thinking I had the problem fixed, I reran; and quickly hit a StackOverFlowException. What the? Ooooh, the if (a == null) was causing that method to be called over and over, recursively until it blew the stack! OK, now how the heck can I test against null without the operator== being called??

After a moment of slight panic, I realized how to get around it. When doing just if (a == null), the Id class’ static overload is being called. The way out of this is to have the object class’ operator== explicitly called by making a simple change:


if ((object)a == null)
{
return ((object)b == null);
}
else
{
return a.Value == b.Value;
}


And that’s it! I now have an overload that handles null checking.