global (C# Reference)
The global contextual keyword, when it comes before the :: operator, refers to the global namespace, which is the default namespace for any C# program and is otherwise unnamed. For more information, see How to: Use the Global Namespace Alias (C# Programming Guide).
Example
The following example shows how to use the global contextual keyword to specify that the class TestApp is defined in the global namespace:
class TestClass : global::TestApp { }