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:: operator - the namespace alias operator

  • Article

Use the namespace alias qualifier :: to access a member of an aliased namespace. You can use the :: qualifier only between two identifiers. The left-hand identifier can be one of a namespace alias, an extern alias, or the global alias. For example:

  • A namespace alias created with a using alias directive:

    using forwinforms = System.Drawing;
using forwpf = System.Windows;

public class Converters
{
    public static forwpf::Point Convert(forwinforms::Point point) => new forwpf::Point(point.X, point.Y);
}

  • An extern alias.

  • The global alias, which is the global namespace alias. The global namespace is the namespace that contains namespaces and types that aren't declared inside a named namespace. When used with the :: qualifier, the global alias always references the global namespace, even if there's the user-defined global namespace alias.

    The following example uses the global alias to access the .NET System namespace, which is a member of the global namespace. Without the global alias, the user-defined System namespace, which is a member of the MyCompany.MyProduct namespace, would be accessed:

    namespace MyCompany.MyProduct.System
{
    class Program
    {
        static void Main() => global::System.Console.WriteLine("Using global alias");
    }

    class Console
    {
        string Suggestion => "Consider renaming this class";
    }
}

    Note

    The global keyword is the global namespace alias only when it's the left-hand identifier of the :: qualifier.

You can also use the . token to access a member of an aliased namespace. However, the . token is also used to access a type member. The :: qualifier ensures that its left-hand identifier always references a namespace alias, even if there exists a type or namespace with the same name.

C# language specification

For more information, see the Namespace alias qualifiers section of the C# language specification.

See also