String.SubString(int  startIndex,int length)   
  startIndex:截取字符串开始的位置     
  length:截取字符串的长度

例子：用户 输入两个数，通过逗号分隔，输入M两个数进行乘运算，输入D两个数进行除法运算。

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

namespace ConsoleApplication1
{
class Program
{

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)
{

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();
double process;
if (input == "M")
{
process = Multiply(param1,param2);
}
else
{
process = Divide(param1,param2);
}
Console.WriteLine("result:{0}", process);
Console.ReadKey();
}
}
}