int im = 20;
           int[] MyList = new int[] {6,15,23,59,80,87,120};
           Array.Sort(MyList);
          for(int i =0;i <MyList.Length -1 ;i++) 
          {
               if(MyList[i]<82 && MyList[i+1]>82){
              MessageBox.Show(i.ToString());
              }
          }

------解决方案--------------------
探讨

int im = 20;
  int[] MyList = new int[] { 6, 15, 23, 59, 80, 87, 120 };

  int 大 = MyList.OrderByDescending(x => x).Where(x => x > im).Min();
  int 小 = MyList.OrderBy(x => x).Where(x => x < im).Ma……


------解决方案--------------------
那你用二分法吧

------解决方案--------------------
二分法，应该不会慢
C# code

        int im = 20;
        int[] MyList = new int[] {6,15,23,59,80,87,120};
        Array.Sort(MyList);
        int index = Array.BinarySearch(MyList, im);
        if (index < 0)
        {
            index = ~index;
        }
        Console.WriteLine(    
                            "Value >= {0} is {1}, value < {0} is {2}.", im, 
                            index == MyList.Length ? "not exists" : MyList[index].ToString(), 
                            index == 0 ? "not exists" : MyList[index - 1].ToString()
                         );