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:
C# Language Specification
For more information, see the following sections in the C# Language Specification:
Reference
C# Operators
Namespace Keywords (C# Reference)
. Operator (C# Reference)
extern alias (C# Reference)
Concepts
C# Programming Guide
Other Resources
C# Reference