What is global::?

It's a sometime-necessary prefix indicating the root namespace.

It's often added to generated code to avoid name clashes with user code.

For example, imagine you had a class called System, but then you wanted to use System.String. You could use global::System.String to differentiate.

I believe the :: comes from C++ where it's used as a namespace separator.

In practice I've never used it, other than in generating code. Note that you can also get around some conflicts via using aliases. For example using String = System.String;

The global contextual keyword, when it comes before the :: operator, refers to the global namespace, which is the default namespace for any C# program and is otherwise unnamed.

The global:: specifier tells the compiler to start looking for the namespace or class starting from the root. You’ll see it in system-generated code so that the code always works. That way if you have a namespace right under your current namespace that is the same as the top level namespace the code is trying to access, there won’t be a conflict.

For example, say you have namespace A and namespace B and namespace B.A if I write code in namespace B.A that needs to reference a class in namespace A, without global:: I have no way of getting to it. If I reference A.classname, the compiler will look for classname in B.A. With global:: I can tell it to look for classname in global::A.classname and it will find classname in the proper location.

The global:: namespace and its identifier is not what most people think. It is not a universal identifier of everything created in an application that lies outside one of your application's defined namespaces and which is attached to some global root.

If you create a class or type outside your top level namespaces you would assume its automatically a part of the GLOBAL namespace and accessible by the global:: identifier in all files in your application or assembly. In fact those names are more often in that file's compiled LOCAL scope only, yet are accessible via the global:: identifier.

If you create a top level class or namespace in an aspx.cs file it is accessible via global:: from the global namespace in that file. But if you type global:: in another file, that class and namespace doesn't exist in the global namespace. If you create that same class or namespace in a class.cs file however, those items are available to all other files via global:: and in the global namespace as well as that files local scope. Why?

It turns out global:: is really a reference to top level LOCAL names under the file's scope as well as GLOBAL names shared by the assembly (like what might be compiled in your App_Code class files in a typical ASP.NET project).

I found this very confusing and not consistent, since global:: implies access to top-level namespaces and types created in the application that are tied to the global namespace. Some like "System" are tied to the global namespace by default in all files, but custom ones may or may not be depending on the scope of that file. That is why the global identifier has a secondary role of resolving references to your local root scope names as well.

You can test this by creating top level namespaces and classes in parts of your application then using global:: to see which ones it can access in the global namespace from different parts of your application and which ones it cannot. The one's it cannot access are clearly assigned to a "local global scope" in that file only, which global:: helps you access in naming conflicts.

global refers to the global namespace, it can be used to solve problems whereby you may redefine types. For example:

class foo
{
    class System
    {

    }

}

If you were to use System where it would be locally scoped in the foo class, you could use:

global::System.Console.WriteLine("foobar");

to access the global namespace.

Example

using System;

class Foo
{
    public void baz()
    {
        Console.WriteLine("Foo 1");
    }
}

namespace Demo
{
    class Foo
    {
        public void baz()
        {
            Console.WriteLine("Foo 2");
        }
    }

    class Program
    {
        protected static global::Foo bar = new global::Foo();

        static void Main(string[] args)
        {
            bar.baz(); // would write Foo 1 to console as it refers to global scope
            Foo qux = new Foo();
            qux.baz(); // would write Foo 2 to the console as it refers to the Demo namespace
        }
    }
}