C#怎么比较三个数的大小并按从大到小的顺序输出?要用if语句.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("请输入三个数：");
double a = Convert.ToDouble (Console .ReadLine ());
double b = Convert.ToDouble(Console.ReadLine());
double c = Convert.ToDouble(Console.ReadLine());
double ? max=null ;
double ? middle =null ;
double ? min=null ;
if (a >= b && b >= c)
{
max = a;
middle = b;
min = c;
}
else
if(a>=c&&c>=b)
{
max = a;
middle = c;
min = b;



}
else
if (b >= a && a >= c)
{
max = b;
middle = a;
min = c;

}
else
if (b >= c && c >= a)
{
max = b;
middle = c;
min = a;

}
else
if (c>= a && a >= b)
{
max = c;
middle = a;
min = b;

}
else
if (c >= b && b >= a)
{
max = c;
middle = b;
min = a;

}
Console.WriteLine ("排序后的序列为（从大到小）：{0}{1}{2}{3}{4}",max ,",",middle,"," ,min);
}
}
}