Why can't you use null as a key for a Dictionary?

Question

Apparently, you cannot use a null for a key, even if your key is a nullable type.

This code:

var nullableBoolLabels = new System.Collect         


        

Answer 1

There is no fundamental reason. HashSet allows null and a HashSet is simply a Dictionary where the key is the same type as the value. So, really, null keys should have been allowed but would be breaking to change it now so we're stuck with it.