Why is the double colon(::) operator required to resolve a namespace conflict? [duplicate]

Question

Please see my below sample program. I have two namespaces containing the same struct. To avoid conflict while using in Main(), I have given the namespaces aliases. While invoking the struct from Main(), I am able to invoke directly through namespace alias, like test.MyStruct. I have another option also using :: operator, like test::MyStruct.

Why is the :: operator required, and where should I use it instead of an alias?

using System;
using test=counter;
using duplicatecounter;

namespace counter
{
    struct MyStruct
    {

    }
}

namespace duplicatecounter
{
    struct MyStruct
    {

    }
}

class Program
{
    public static void Main()
    {
        test.MyStruct a = new test.MyStruct();
        test::MyStruct a1 = new test::MyStruct();
    }
}

Answer 1

It is mainly needed when someone wrote code without consideration of code being used. I.e. duplicate classes in namespaces that are expected to be used together or hiding namespaces.

MSDN sample shows one case in Use the Global Namespace Alias :

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()
    {
        // The following line causes an error. It accesses TestApp.Console, 
        // which is a constant. 
        //Console.WriteLine(number);

        global::System.Console.WriteLine(number); // ok

    }
}

Answer 2

the :: operator doing the same like namespace. ,but the :: operator is used to look up identifiers. It is always positioned between two identifiers

example :

global::System.Console.WriteLine("Hello World");

a good example explained here : http://msdn.microsoft.com/en-us/library/c3ay4x3d.aspx