Result of calling IEquatable.Equals(T obj) when this == null and obj == null?

Question

What should IEquatable.Equals(T obj) do when this == null and obj == null?

1) This code is generated

Answer 1

leppie is right. Just to elaborate on his answer (and confirm his suspicion that F# doesn't guarantee this != null): discriminated unions may be marked with the attribute [] allowing cases to be represented by the value null. Option<'T> is such a type. The None case is represented by null at run-time. (None : option).Equals(None) is syntactically valid. Here's a fun example:

[]
type Maybe<'T> =
  | Just of 'T
  | Nothing
  []
  member this.ThisIsNull() = match this with Nothing -> true | _ -> false

Decompiling ThisIsNull with Reflector shows

public bool ThisIsNull()
{
    return (this == null);
}

And the result:

Nothing.ThisIsNull() //true