(C#) Problems when overloading the == operator [duplicate]

Question

I overloaded the == operator on my class as follows:

public static bool operator ==(Table pt1, Table pt2) {
    return Compare(pt1, pt2) == 0 && pt1.TableName == pt2.TableName;
}

Compare will work just as the strcmp does in c++, returning an integer. Problem is that if I do an if (MY_CLASS == null), it will call my == operator, and thus my Compare function. What is the alternatiev? to put an check on pt1 and pt2 to see if they are null? Or just in pt2?

Answer 1

You should checkout Microsoft's guidelines for implementing the '==' operator and also for overriding 'Equals()'.

Adapting their example you'd want something like:

public static bool operator ==(Table a, Table b)
{
    // If both are null, or both are same instance, return true.
    if (System.Object.ReferenceEquals(a, b))
    {
        return true;
    }

    // If one is null, but not both, return false.
    if (((object)a == null) || ((object)b == null))
    {
        return false;
    }

    // Return true if the fields match:
    return Compare(a, b) == 0 && a.TableName == b.TableName;
}

Answer 2

You will need the null checks to get the correct behaviour. The clearest way to add this check in my opinion is to call object.ReferenceEquals(x, null), since it is a straight non-polymorphic method call and presumably fairly efficient.