Calling an extension method on a null instance

Today I learned that you can call an extension method on a null instance of the type. I had always assumed without thinking about it too hard, that the reason string.IsNullOrEmpty isn’t defined as an extension method in the framework is because you would get NullReferenceException.

But if we were to define an extension method like public static bool IsNullOrEmpty(this string s), and call it like if(s.IsNullOrEmpty()) this is just syntactic sugar for if(StringExtensions.IsNullOrEmpty(s)) and therefore it’s safe to call on a null instance.

Apparently the argument against this is that it could be confusing or counterintuitive when looking at the code if you think it’s an instance method. I personally fall on the side of adding convenient extension methods like this - if nothing else to add a NotNullOrEmpty method.

To me I’ve always found it very easy to miss the ! operator in a conditional and so avoiding those makes the condition much easier to read.

I much perfer:
if(userName.NotNullOrEmpty)
over
if(!string.IsNullOrEmpty(userName))