How would you share the same object between two other objects? For instance, I'd like something in that flavor:

class A
{
   private string foo_; // It could be any other class/struct too (Vector3, Matrix...)

   public A (string shared)
   {
       this.foo_ = shared;
   }

   public void Bar()
   {
     this.foo_ = "changed";
   }
}

...
// inside main
string str = "test";
A a = new A(str);

Console.WriteLine(str); // "test"
a.Bar();
Console.WriteLine(str); // I get "test" instead of "changed"... :(

I read there is some ref/out stuff, but I couldn't get what I'm asking here. I could only apply some changes in the methods scope where I was using ref/out arguments... I also read we could use pointers, but is there no other way to do it?