c#中的代理delegate，怎么用？什么时候用？他对方法的密封性体现在什么地方？

using System;

namespace Ch06Ex05
{
/// <summary>
/// Summary description for Class1.
/// </summary>
class Class1
{
delegate double processDelegate(double param1, double param2);

static double Multiply(double param1, double param2)
{
return param1 * param2;
}

static double Divide(double param1, double param2)
{
return param1 / param2;
}

static void Main(string[] args)
{
processDelegate process;
Console.WriteLine("Enter 2 numbers separated with a comma:");
string input = Console.ReadLine();
int commaPos = input.IndexOf(',');
double param1 = Convert.ToDouble(input.Substring(0, commaPos));
double param2 = Convert.ToDouble(input.Substring(commaPos + 1,
input.Length - comm