extern alias TheAliasYouGaveTheReference

TheAliasYouGaveTheReference::Acme.Foo f = new
    TheAliasYouGaveTheReference::Acme.Foo();

using System;

namespace Acme
{
    public class Foo
    {
        public void Bar()
        {
            Console.WriteLine("Bar");
        }
    }
}

using System;

namespace Acme
{
    public class Foo
    {
        public void Bar()
        {
            Console.WriteLine("Bar");
        }

        public void Goo()
        {
            Console.WriteLine("Goo");
        }
    }
}

Acme.Foo f = new Acme.Foo();

extern alias FooVersion1;

FooVersion1::Acme.Foo f = new FooVersion1::Acme.Foo();
f.Bar();

Acme.Foo f2 = new Acme.Foo();
f2.Goo();