C#函数参数传递 ref关键字

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication2
{
    class Program
    {
        static void Swap(ref int x,ref int y)
        {
            int temp = x;
            x = y;
            y = temp;
        }

        static void Main(string[] args)
        {
            int a = 40;
            int b = 80;
            Console.WriteLine("交换前: a = {0},b = {1}",a,b);
            Swap(ref a,ref b);
            Console.WriteLine("交换后: a = {0},b = {1}",a,b);
            Console.ReadKey();
        }
    }
}

交换前: a = 40,b = 80
交换后: a = 80,b = 40
ref修饰的变量必须要赋值