title: In C# where does the key words come from if "using system" is commented?
body: <p>For example, we know that "int" type in C# is nothing but a structure which is actually System.Int32. If that so, then if "using system;" is commented in a program, then int type should not be able to use. But still int type can be used. My question is, from where these types are coming from?</p>
  
  <pre><code>//using System;
  class Program
  {
      static void Main() {
           int x = 0;  // it still work, though int is under System namespace, why??
      }
  }
  </code></pre>