OK; that is going to break when nulls are involved, so simply deal with
those separately (see below); most of the code is AreEqual - first we handle
"are they the same instance" (which also handles "are they both null") - if
so, must be the same. Then if *either* is null (by itself) then they can't
be equal. So now we've excluded every case involving null; *now* we can
worry about the field equality.

Note also that I've made Equals() consistent with these rules (reports false
if it is compared to something that isn't a Slot), and I've provided a
meaningful hash implementation (so you can use the object in a
dictionary/hashtable).

Does that work any better? To be honest, I'm not 100% sure how far you got,
so rather than me prattle on, if you have any specific questions, please
ask?

Marc

using System;
class Slot : IEquatable<Slot>
{
   private int startTimeId, endTimeId;
   public override int GetHashCode()
   {
       return (17 * startTimeId.GetHashCode()) + endTimeId.GetHashCode();
   }

   public override bool Equals(object obj)
   {
       Slot slot = obj as Slot;
       return slot == null ? false : AreEqual(this, slot);
   }
   public bool Equals(Slot slot)
   {
       return AreEqual(this, slot);
   }

   private static bool AreEqual(Slot s1, Slot s2)
   {
       if (ReferenceEquals(s1, s2)) return true;
       if (ReferenceEquals(s1, null)
           || ReferenceEquals(s2, null)) return false;

       return s1.startTimeId == s2.startTimeId
           && s1.endTimeId == s2.endTimeId;
   }
   public static bool operator ==(Slot s1, Slot s2)
   {
       return AreEqual(s1, s2);
   }
   public static bool operator !=(Slot s1, Slot s2)
   {
       return !AreEqual(s1, s2);
   }
}

ReferenceEquals *is* what you're after, so could you post a short but
complete program which demonstrates the problem, attempting to use
ReferenceEquals?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by "short but complete program".

(I have to say that overloading == but leaving Equals and GetHashCode
doing their original comparisons is somewhat confusing behaviour.)

see inline

    if ( obj == null || ! (obj is Slot))
       return false; // "Slot" (this) against non-Slot: fail

    Slot other = (Slot)obj;
    // and now compare similar to the == operator

  if (s1 == null && s2 == null)
     return true; // both null: so equal
  if (s1 == null || s2 == null)
     return false; // one is null, the other not: not equal

Hans Kesting