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The namespace alias qualifier operator.
The namespace alias qualifier (::) is used to look up identifiers. It is always placed between two identifiers, as in this example:
global::System.Console.WriteLine("Hello World");
The namespace alias qualifier can be global. This invokes a lookup in the global namespace, rather than an aliased namespace.
For an example of using the :: operator, see the following section:
How to: Use the Namespace Alias Qualifier (C# Programming Guide)
For more information, see the following sections in the C# Language Specification:
20.9.5 Simple names
25.3 Namespace alias qualifiers