:: Operator (C# Reference)
The namespace alias qualifier (::) is used to look up identifiers. It is always positioned between two identifiers, as in this example:
The namespace alias qualifier can be global. This invokes a lookup in the global namespace, rather than an aliased namespace.
For More Information
For an example of how to use the :: operator, see the following section:
C# Language Specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.