Sometimes, it can be useful to make an extension method specifically for a single block of code.  Unfortunately, since extension methods cannot appear in nested classes, there is no obvious way to do that.

Instead, you can create a child namespace containing the extension method.  In order to limit the extension method’s visibility to a single method, you can put that method in a separate namespace block.  This way, you can add a using statement to that namespace alone.

For example:

namespace Company.Project {
    partial class MyClass {
        ...
    }
}
namespace Company.Project {
    using MyClassExtensions;
    namespace MyClassExtensions {
        static class Extensions {
            public static string Name<T>(this T obj) {
                if (default(T) == null && Equals(obj, default(T)))
                    return "(null " + typeof(T) + ")";
                return obj.GetType() + ": " + obj.ToString() 
                     + "{declared as " + typeof(T) + "}";
            }
        }
    }
    partial class MyClass {
        void DoSomething() {
            object x = new DateTime();
            string name = x.Name();
        }
    }
}

Since the using MyClassExtensions statement appears inside the second namespace block, the extension methods are only visible within that block.  Code that uses these extension method can appear in this second block, while the rest of the class can go in the original namespace block without the extension methods.

This technique should be avoided where possible, since it leads to confusing and non-obvious code.  However, there are situations in which this can make some code much more readable.