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 description for Class1.
///

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 - commaPos - 1));
Console.WriteLine("Enter M to multiply or D to divide:");
input = Console.ReadLine();
if (input == "M")
process = new processDelegate(Multiply);
else
process = new processDelegate(Divide);
Console.WriteLine("Result: {0}", process(param1, param2));
}

}
}