Is there a particular reason why string.IsNullOrEmpty() is not an extension method?

Question

C# has extension methods .

Some static methods of the string class can be rewritten as extension methods eg String.IsNullOrEmpty .

Is there a reason why there are no default extension method for the string class that provide the same functionality as the static class?

Answer 1

Yes. A really good one.

It PREDATES the existence of extension methods. It simply was there first.

There are quite some similar scenarios around. Lots of API taking a typeof but not having a generic version, ie - all predating generics.

Answer 2

Look at the string.IsNullOrEmpty documentation (specifically, the "Applies to" section).

This method existed since .NET Framework 2.0.

If you look at the history of C# from the official docs , you can simply find that extension methods came with C# 3.0, which was around .NET Framework 3.5.

Note: Don't mix between C# versions and the .NET versions.

Answer 3

Another reason is the method would fail if the string reference was null. Consider something like this:

string x = null;
if (x.isNullOrEmpty()) ...

This call would obviously cause an error.