为什么字符串的行为类似于ValueType

执行这段代码后，我感到困惑，其中的字符串似乎就像值类型一样.我想知道赋值运算符是否正在对字符串的相等运算符之类的值进行运算.

I was perplexed after executing this piece of code, where strings seems to behave as if they are value types. I am wondering whether the assignment operator is operating on values like equality operator for strings.

这是我测试此行为的代码段.

Here is the piece of code I did to test this behavior.

using System;

namespace RefTypeDelimma
{
    class Program
    {
        static void Main(string[] args)
        {
            string a1, a2;

            a1 = "ABC";
            a2 = a1; //This should assign a1 reference to a2
            a2 = "XYZ";  //I expect this should change the a1 value to "XYZ"

            Console.WriteLine("a1:" + a1 + ", a2:" + a2);//Outputs a1:ABC, a2:XYZ
            //Expected: a1:XYZ, a2:XYZ (as string being a ref type)

            Proc(a2); //Altering values of ref types inside a procedure 
                      //should reflect in the variable thats being passed into

            Console.WriteLine("a1: " + a1 + ", a2: " + a2); //Outputs a1:ABC, a2:XYZ
            //Expected: a1:NEW_VAL, a2:NEW_VAL (as string being a ref type)
        }

        static void Proc(string Val)
        {
            Val = "NEW_VAL";
        }
    }
}

在上面的代码中，如果我使用自定义类而不是字符串，则会得到预期的行为.我怀疑这与字符串不变性有关吗?

In the above code if I use a custom classes instead of strings, I am getting the expected behavior. I doubt is this something to do with the string immutability?

对此表示欢迎.