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

Question

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

1) This code is generated by F# compiler when implementing IEquatable<T>. You can see that it returns true when both objects are null:

    public sealed override bool Equals(T obj)
    {
        if (this == null)
        {
            return obj == null;
        }
        if (obj == null)
        {
            return false;
        }

        // Code when both this and obj are not null.
    }

2) Similar code can be found in the question "in IEquatable implementation is reference check necessary" or in the question "Is there a complete IEquatable implementation reference?". This code returns false when both objects are null.

    public sealed override bool Equals(T obj)
    {
        if (obj == null)
        {
            return false;
        }

        // Code when obj is not null.
    }

3) The last option is to say that the behaviour of the method is not defined when this == null.

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 [<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>] 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<int>).Equals(None) is syntactically valid. Here's a fun example:

[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
type Maybe<'T> =
  | Just of 'T
  | Nothing
  [<CompilationRepresentation(CompilationRepresentationFlags.Instance)>]
  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