SolucionConflictoNamespaceClase.cs

namespace N
{
    class A
    {
        static void Main()
        {
            System.Console.WriteLine (new A.B());
            System.Console.WriteLine (new global::A.B());
        }

        public class B
        {
            // ...
        }
    }
}

namespace A
{
    class B
    {
        // ...
    }
}