C#传递值/参考？

using System;

class Program
{
    static void Main()
    {
        int x = 10;
        int y = 20;

        Console.WriteLine("Before: x = {0}, y = {1}", x, y);

        // 值传递
        Add(x, y);
        Console.WriteLine("After Add(x, y): x = {0}, y = {1}", x, y);

        // 引用传递
        AddRef(ref x, ref y);
        Console.WriteLine("After AddRef(ref x, ref y): x = {0}, y = {1}", x, y);
    }

    static void Add(int a, int b)
    {
        a = a + b;
        b = a - b;
    }

    static void AddRef(ref int a, ref int b)
    {
        a = a + b;
        b = a - b;
    }
}

Before: x = 10, y = 20
After Add(x, y): x = 10, y = 20
After AddRef(ref x, ref y): x = 30, y = 10