理解：引用类型，string类型(字符串驻留)，值类型

class Class1  {
       private string str = "Class1.str";
       private int i = 0;
       static void StringConvert(string str)  {
           str = "string being converted.";
       }
       static void StringConvert(Class1 c)  {
           c.str = "string being converted.";
       }
       static void Add(int i)  {
           i++;
       }
       static void AddWithRef(ref int i)  {
           i++;
       }
       static void Main()  {
           int i1 = 10;
           int i2 = 20;
           string str = "str";
           Class1 c = new Class1();
           Add(i1);
           AddWithRef(ref i2);
           Add(c.i);
           StringConvert(str);        
           StringConvert(c);
           Console.WriteLine(i1);
           Console.WriteLine(i2);
           Console.WriteLine(c.i);
           Console.WriteLine(str);
           Console.WriteLine(c.str);
       } 
    }