How do I override the equals operator == for an interface in C#?

Question

I have defined the following interface:

public interface IHaveAProblem
{
    string Issue { get; set; }
}

And here is the implementation of IHaveAProblem:

public class SomeProblem : IHaveAProblem
{
    public string Issue { get; set; }

    public override bool Equals(object obj)
    {
        SomeProblem otherObj = obj as SomeProblem;

        if (otherObj == null)
        {
            return false;
        }

        return this.Issue == otherObj.Issue;
    }

    public override int GetHashCode()
    {
        return base.GetHashCode();
    }
    
    public static bool operator ==(SomeProblem rhs, SomeProblem lhs)
    {
        // Null check
        if (Object.ReferenceEquals(rhs, null) || Object.ReferenceEquals(lhs, null))
        {
            if (Object.ReferenceEquals(rhs, null) && Object.ReferenceEquals(lhs, null))
            {
                // Both are null.  They do equal each other
                return true;
            }

            // Only 1 is null the other is not so they do not equal
            return false;
        }

        return rhs.Equals(lhs);
    }

    public static bool operator !=(SomeProblem rhs, SomeProblem lhs)
    {
        // Null check
        if (Object.ReferenceEquals(rhs, null) || Object.ReferenceEquals(lhs, null))
        {
            if (Object.ReferenceEquals(rhs, null) && Object.ReferenceEquals(lhs, null))
            {
                // Both are null.  They do equal each other
                return false;
            }

            // Only 1 is null the other is not so they do not equal
            return true;
        }

        return !rhs.Equals(lhs);
    }
}

When I use the object, I can get the correct results for the == compare:

SomeProblem firstTest = new SomeProblem()
    {
        Issue = "Hello World"
    };

SomeProblem secondTest = new SomeProblem()
    {
        Issue = "Hello World"
    };

// This is true
bool result = firstTest == secondTest;

However, when I try to compare the interfaces, it is doing a memory compare rather than the operator == on SomeProblem:

IHaveAProblem firstProblem = new SomeProblem()
    {
        Issue = "Hello World"
    };

IHaveAProblem secondProblem = new SomeProblem()
    {
        Issue = "Hello World"
    };

Is it possible to have the interface use the == on SomeProblem rather than a memory compare?

I know I can do a firstProblem.Equals(secondProblem) and get the proper results. However, I am creating a framework and I will not know how it is used in the end. I thought == would work correctly.

Answer 1

The operator == is static. You cannot define static methods for interfaces in C#. Also, for all operators at least one of the argument types needs to be of the same type as the class it is defined in, therefore: No operator overloading for interfaces :(

What you CAN do is use an abstract class instead - and define the operator there. Again, the operator may NOT be virtual (since static methods cannot be virtual...)