问题：在没有任何数据时进行无效的读取尝试

  private void button1_Click(object sender, EventArgs e)
        {
          
            //连接语句 
            string connTeststr = "Data Source=.;Initial Catalog=TEST;Integrated Security=True";
            string connStustr = "Data Source=.;Initial Catalog=student;Integrated Security=True";
            //SqlConnection建立连接
            SqlConnection connTest = new SqlConnection(connTeststr);
            SqlConnection connStu = new SqlConnection(connStustr);
            //打开连接
            connTest.Open();
            connStu.Open();

            //连接测试
            if (connTest.State == ConnectionState.Open)
            { label1.Text = "TEST连接成功"; }
            else
            { label1.Text = "连接失败"; }
            if (connStu.State == ConnectionState.Open)
            { label2.Text = "STU连接成功"; }
            else
            { label2.Text = "连接失败"; }

            //上面连接 已经成功
            //下面该DATASET 测试

            SqlCommand Cmd = new SqlCommand("select 学号 from XSB", connStu);
            SqlDataReader reader = Cmd.ExecuteReader();
            
            string bkstr = reader.GetString(0);


           
            textBox1.Text = bkstr;

            reader.Close();



        }

[解决办法]
sql没有查询到数据
先判断下
if(reader.Read())
{
string bkstr = reader.GetString(0);
}