gridview 里面dropdownlist 绑定 奇怪的问题

Question

    <asp:GridView ID="GridView_stu" runat="server" CssClass = "grid"  
        AutoGenerateColumns="False" onrowdatabound="GridView_stu_RowDataBound"  DataKeyNames ="courseID"
              Height="100%">
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:CheckBox ID="CheckBox1" runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:BoundField DataField="courseName" HeaderText="课题名称" />
            <asp:BoundField DataField="style" HeaderText="课题类型" />
            <asp:BoundField DataField="stuName" HeaderText="学生姓名" />
            <asp:BoundField DataField="stuID" HeaderText="学号" />
            <asp:BoundField DataField="teachName" HeaderText="指导教师" /> 
            <asp:TemplateField HeaderText="工号">
                <ItemTemplate>
                    <asp:DropDownList ID="DropDownList1" runat="server">
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="状态">
                <ItemTemplate>
                    <asp:Label ID="Lbl_state" runat="server" Text='<%# bind("state") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>        protected void Page_Load(object sender, EventArgs e)
    {
        stu_bind();
    }
    
   protected void stu_bind()
   {
       SQL sql = new SQL();
       string sqlstr = "select * from stuCourse";
       int recorder = Convert.ToInt32(((DataTable)sql.mytable(sqlstr)).Rows.Count);
       this.AspNetPager1.RecordCount = recorder;
       SqlConnection myconn = DB.creatconn();
       SqlDataAdapter myda = new SqlDataAdapter(sqlstr, myconn);
       DataSet myds = new DataSet();
       myda.Fill(myds, (this.AspNetPager1.CurrentPageIndex - 1) * AspNetPager1.PageSize, AspNetPager1.PageSize, "table");       this.GridView_stu.DataSource = myds.Tables["table"].DefaultView;
       this.GridView_stu.DataBind();       for (int i = 0; i <= myds.Tables["table"].Rows.Count - 1;i ++)
       {
           if (myds.Tables["table"].Rows[i]["teachName"].ToString() == "" || myds.Tables["table"].Rows[i]["teachName"].ToString() == null)
           {
               CheckBox cbox = (CheckBox)GridView_stu.Rows[i].FindControl("CheckBox1");
               cbox.Enabled = false;
           }
           else
           {
               CheckBox cbox = (CheckBox)GridView_stu.Rows[i].FindControl("CheckBox1");
               cbox.Enabled = true;
               SQL sqlID = new SQL();
               string sqlstrID = "select teach_id from teacher where teach_name = '" + myds.Tables["table"].Rows[i]["teachName"].ToString() + "'";
               DataTable tableid = (DataTable)sqlID.mytable(sqlstrID);
               DropDownList ddl = (DropDownList)GridView_stu.Rows[i].FindControl("DropDownList1 ");             
               ddl.DataSource = tableid;
               ddl.DataTextField = "teach_id";
               ddl.DataValueField = "teach_id";
               ddl.DataBind();
           }
       }
   }
ddl.DataSource = tableid；
异常详细信息: System.NullReferenceException: 未将对象引用设置到对象的实例。很郁闷，tableid里面是有数据的  但是就是会出现上面这种错误  ，很奇怪的问题！！！！

Answer 1

你调试看下 ddl 这个对象有没有为null

Answer 2

感谢啊  真的是ddl 这个对象有没有为null 谢谢了