C# Can I make a namespace accessible everywhere in my dot net web site for code-behind and classes

C# Can I make a namespace accessible everywhere in my dot net web site for
code-behind and classes

I have an extension method for String that I want to be available on every
code behind and class in my solution. The method is sitting in a
particular namespace. I'd like everywhere to have access to that namespace
without me having to include it in every class.
I've used the "namespaces" tag in the web config to successfully include
it on every aspx page, but this does not make it accessible on code behind
or elsewhere.
So, is there a similar way to include a namespace everywhere?