What is the usage of global:: keyword in C#?

What is the usage of global::开发者_运维知识库 keyword in C#? When must we use this keyword?

Technically, global is not a keyword: it's a so-called "contextual keyword". These have special meaning only in a limited program context and can be used as identifiers outside that context.

global can and should be used whenever there's ambiguity or whenever a member is hidden. From here:

class TestApp
{
    // Define a new class called 'System' to cause problems.
    public class System { }

    // Define a constant called 'Console' to cause more problems.
    const int Console = 7;
    const int number = 66;

    static void Main()
    {
        // Error  Accesses TestApp.Console
        Console.WriteLine(number);
        // Error either
        System.Console.WriteLine(number);
        // This, however, is fine
        global::System.Console.WriteLine(number);
    }
}

Note, however, that global doesn't work when no namespace is specified for the type:

// See: no namespace here
public static class System
{
    public static void Main()
    {
        // "System" doesn't have a namespace, so this
        // will refer to this class!
        global::System.Console.WriteLine("Hello, world!");
    }
}