何时使用ref和out

所以我正在为我的大学课堂做一个非常简单的实验室，遇到了一些问题。我的老师没有彻底解释你什么时候用ref，什么时候用out。任务是将预先编写的方法更改为使用ref，然后确保它运行，然后将其更改为使用out。我记下了ref部分，但我该如何重写方法以使用它呢？以下是程序

using System;
static class Program
{
    /// <summary>
    /// Purpose: Entry point to your C# program
    /// </summary>
    static void Main()
    {
        int iVal1 = 5;
        int iVal2 = 7;
        //Call the Swap method with two arguments
        Swap(ref iVal1, ref iVal2);
        Console.WriteLine("Swapped values first {0:D} second {1:D}", iVal1, iVal2);
        Console.WriteLine("Press Enter to continue ...");
        Console.ReadLine();
    }//End Main()
    /// <summary>
    /// Purpose: To swap the two parameters passed to this method
    /// </summary>
    /// <param name="num1">num1 int, first number</param>
    /// <param name="num2">num2 int, second number</param>
    static public void Swap(ref int num1, ref int num2)
    {
        int tempInt = num1;
        num1 = num2;
        num2 = tempInt;
    }
}//End class Program