ArrayList arrP = new ArrayList();
//init arrP;//题库中题目的ID号组

ArrayList arrT = new ArrayList();
Random rand = new Random();
int i = 0;
while(i<20)
{
  int index = rand.Next(0,500);
  if(!arrT.Contains(arrp[index]) //也可只存index,后期再取值
  {
    arrT.Add(arrp[index]);
    i++;
  }
}