IEqualityComparer for Value Objects

Question

I have an immutable Value Object, IPathwayModule, whose value is defined by:

• (int) Block;
• (Entity) Module, identified by (string) ModuleId;
• (enum) Status; and
• (entity) Class, identified by (string) ClassId - which may be null.

Here's my current IEqualityComparer implementation which seems to work in a few unit tests. However, I don't think I understand what I'm doing well enough to know whether I am doing it right. A previous implementation would sometimes fail on repeated test runs.

private class StandardPathwayModuleComparer : IEqualityComparer<IPathwayModule>
{
    public bool Equals(IPathwayModule x, IPathwayModule y)
    {
        int hx = GetHashCode(x);
        int hy = GetHashCode(y);
        return hx == hy;
    }

    public int GetHashCode(IPathwayModule obj)
    {
        int h;
        if (obj.Class != null)
        {
            h = obj.Block.GetHashCode() + obj.Module.ModuleId.GetHashCode() + obj.Status.GetHashCode() + obj.Class.ClassId.GetHashCode();
        }
        else
        {
            h = obj.Block.GetHashCode() + obj.Module.ModuleId.GetHashCode() + obj.Status.GetHashCode() + "NOCLASS".GetHashCode();
        }
        return h;
    }
}

IPathwayModule is definitely immutable and different instances with the same values should be equal and produce the same HashCode since they are used as items within HashSets.

I suppose my questions are:

• Am I using the interface correctly in this case?
• Are there cases where I might not see the desired behaviour?
• Is there any way to improve the robustness, performance?
• Are there any good practices that I am not following?

Answer 1

Thanks to all who responded. I have aggregated the feedback from everyone who responded and my improved IEqualityComparer now looks like:

private class StandardPathwayModuleComparer : IEqualityComparer<IPathwayModule>
{
    public bool Equals(IPathwayModule x, IPathwayModule y)
    {
        if (x == y) return true;
        if (x == null || y == null) return false;

        if ((x.Class == null) ^ (y.Class == null)) return false;

        if (x.Class == null) //and implicitly y.Class == null
        {
            return x.Block.Equals(y.Block) && x.Status.Equals(y.Status) && x.Module.ModuleId.Equals(y.Module.ModuleId);
        }
        return x.Block.Equals(y.Block) && x.Status.Equals(y.Status) && x.Module.ModuleId.Equals(y.Module.ModuleId) && x.Class.ClassId.Equals(y.Class.ClassId);
    }
    public int GetHashCode(IPathwayModule obj)
    {
        unchecked {
            int h = obj.Block ^ obj.Module.ModuleId.GetHashCode() ^ (int) obj.Status;
            if (obj.Class != null)
            {
               h ^= obj.Class.ClassId.GetHashCode();
            }
            return h;
        }
    }
}