关于 string和StringBuilder的区别参考MSDN。本文用程序演示它们在内存中的区别，及其因此其行为不同。

//Demo  string memory model

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {

string a = "1234";

string b = a;//a,and b point to the same address

Console.WriteLine(a);

Console.WriteLine(b);

a = "5678";

Console.WriteLine(a);

Console.WriteLine(b);//That b's value is not changed means string's value cann't be changed

Console.ReadKey();
        } 
            
    }
}

output:

1234

1234

5678;change a's value,b's value is not changed

1234

//Demo StringBuilder's memory model

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
  
            StringBuilder a = new StringBuilder("1234");
            StringBuilder b = new StringBuilder();
            b = a;
            a.Clear();
            a.Append("5678");
            Console.WriteLine(a);
            Console.WriteLine(b);
            Console.ReadKey();
        }

}
}

output:

5678

5678