c#控制台程序循环问题

Question

Console.WriteLine("Coffee sizes: 1=Small 2=Medium 3=Large");//输出并换行
        Console.Write("Please enter your selection: ");//输出不换行
        string s = Console.ReadLine();
        if (s!="")
        {
            int n = int.Parse(s);//强制转换成int
            int cost = 0;
            switch (n)
            {
                case 1:
                    cost += 25;
                    break;
                case 2:
                    cost += 50;
                    break;
                case 3:
                    cost += 50;
                    goto case 1;//...
                default:
                    Console.WriteLine("Invalid selection. Please select 1, 2, or 3.");//无效信息
                    break;
            }
            if (cost != 0)
            {
                Console.WriteLine("Please insert {0} {1}cents.", "$", cost);//{}..
                Console.ReadLine();//从标准输入到下一行字符
            }
            Console.WriteLine("Thank you for your business.");
            Console.ReadLine();
        }
        else
        {
            Console.WriteLine("please don't joke me!can you select 1, 2, or 3?");
            Console.ReadLine();
        }比如运行后，我什么都没输入，直接回车，执行else内容则提示please don't joke me!can you select 1, 2, or 3?，这个时候我怎么才能回到开头Console.Write("Please enter your selection: ");循环呢？感觉大一C语言有过这个问题，貌似忘了。。

Answer 1

char c=getchar();
while(c!='q')
{
    swich(c)
   {
   case 'a':
      dosomthing;
      break;
   case 'b':
      dosomthing;
      break;
   }
   c=getchar();
}

Answer 2

while(true)
{
   Console.Write("Please enter your selection: ");//输出不换行
        string s = Console.ReadLine();
        if (s!="")
        {
            int n = int.Parse(s);//强制转换成int
            int cost = 0;
            switch (n)
            {
                case 1:
                    cost += 25;
                    break;
                case 2:
                    cost += 50;
                    break;
                case 3:
                    cost += 50;
                    goto case 1;//...
                default:
                    Console.WriteLine("Invalid selection. Please select 1, 2, or 3.");//无效信息
                    break;
            }
            if (cost != 0)
            {
                Console.WriteLine("Please insert {0} {1}cents.", "$", cost);//{}..
                Console.ReadLine();//从标准输入到下一行字符
            }
            Console.WriteLine("Thank you for your business.");
            Console.ReadLine();
        }
        else
        {
            Console.WriteLine("please don't joke me!can you select 1, 2, or 3?");
            Console.ReadLine();
        }
}