why string.IsNullOrEmpty() is created?

Question

If string.Empty != null why string.IsNullOrEmpty() is created?

I just want to say that:
if null and string.Empty are different to each other.

• why string.IsNull(); and string.IsEmpty(); separate methods does not exist.
• why a combined method string.IsNullOrEmpty() exists?

Answer 1

• string.IsNull doesn't exist because you'd just check for the reference being null
• string.IsEmpty doesn't exist because you can easily compare for equality with "" or for a length of 0

• string.IsNullOrEmpty exists because it's simpler to write the single method call than use

     if (text == null || text.Length == 0)
  

  (or the inverse, of course).

Each of the individual checks can be done simply on its own, but it's convenient to have a combination of the two.