c#中自增与自减运算怎么理解？

Question

using System;

namespace OperatorsAppl
{
class Program
{
static void Main(string[] args)
{
int a = 21;
int b = 10;
int c;

        c = a + b;
        Console.WriteLine("Line 1 - c 的值是 {0}", c);
        c = a - b;
        Console.WriteLine("Line 2 - c 的值是 {0}", c);
        c = a * b;
        Console.WriteLine("Line 3 - c 的值是 {0}", c);
        c = a / b;
        Console.WriteLine("Line 4 - c 的值是 {0}", c);
        c = a % b;
        Console.WriteLine("Line 5 - c 的值是 {0}", c);
        c = a++;
        Console.WriteLine("Line 6 - c 的值是 {0}", c);
        c = a--;
        Console.WriteLine("Line 7 - c 的值是 {0}", c);
        Console.ReadLine();
    }
}

}

运行结果为：
Line 1 - c 的值是 31
Line 2 - c 的值是 11
Line 3 - c 的值是 210
Line 4 - c 的值是 2
Line 5 - c 的值是 1
Line 6 - c 的值是 21
Line 7 - c 的值是 22

为什么：line6-c的值是21而不是22
line7-c的值是22而不是20

Answer 1

a++和a--是先赋值后运算。++a和--a是先运算后赋值。后面那几行相当于

         c = a;//c=21
                a=a+1;//a=22
        Console.WriteLine("Line 6 - c 的值是 {0}", c);//输出21
        c = a//c=22,因为a=22
                a=a-1;//a=21
        Console.WriteLine("Line 7 - c 的值是 {0}", c);//输出22