double a;
            string b;
            a = double.Parse(txt100.Text.Trim());
            if (a>100 || a<0);
            {
                MessageBox.Show("输入的分数有误！","错误",MessageBoxButtons.OK
                  ,MessageBoxIcon.Error);
              return;
            }
            if (a <= 60 )
                b = "差";
            else if (a <= 70)
                b = "良";
            else if (a <= 80)
                b = "中";
            else if (a <= 99)
                b = "优";
            else
                b = "完美";
            txt5.Text = b;
   这个提示说有错误 我真心不知道哪里错了 大家过来帮为看看到底哪里错了