GridView更新时错误，希望有人帮助。困扰很久了！

Question

页面代码：
<asp:GridView ID="grv1" runat="server" AutoGenerateColumns="False" 
            onrowcancelingedit="grv1_RowCancelingEdit" onrowdatabound="grv1_RowDataBound" 
            onrowediting="grv1_RowEditing" onrowupdating="grv1_RowUpdating">
    <Columns>
    <asp:BoundField HeaderText="状态" DataField="state" />
        <asp:CommandField ShowEditButton="True" />
    </Columns>
</asp:GridView>
====================================================================
后台代码：
protected void grv1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowIndex==grv1.EditIndex)
        {            
            e.Row.Cells[0].Controls.Clear();
            DropDownList ddl = new DropDownList();
            ddl.Items.Add("已维修");
            ddl.Items.Add("维修中");
            ddl.Items.Add("未维修");
            e.Row.Cells[0].Controls.Add(ddl); 
        }
    }protected void grv1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].Cells[0].Controls[0]; // 运行到这里出错了，提示不能将TextBox转换为DropDownList。
        ......
        grv1.EditIndex = -1;
        BindData();
    }

Answer 1

DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].Cells[0].Controls[0]; // 运行到这里出错了，提示不能将TextBox转换为DropDownList。 
=======>
使用这种这种插入方法~~
this.DDL_Cate.Items.Insert(0, new ListItem("请选择", "-1"));

Answer 2

 DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].Cells[0].Controls[0]; // 运行到这里出错了，提示不能将TextBox转换为DropDownList。 是CELLs[0]找索引错了还是Controls[0]找索引错了

Answer 3

DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].Cells[0].Controls[0];你那个Control肯定是TextBox，所以弄错了最好
DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].FindControl("ddlID");

Answer 4

protected void grv1_RowDataBound(object sender, GridViewRowEventArgs e) 
    { 
        if (e.Row.RowIndex==grv1.EditIndex) 
        {            
            e.Row.Cells[0].Controls.Clear(); 
            DropDownList ddl = new DropDownList(); 
            ddl.Id="aaaaaa1";
            ddl.Items.Add("已维修"); 
            ddl.Items.Add("维修中"); 
            ddl.Items.Add("未维修"); 
            e.Row.Cells[0].Controls.Add(ddl); 
        } 
    } protected void grv1_RowUpdating(object sender, GridViewUpdateEventArgs e) 
    { 
        DropDownList ddl = (DropDownList)grv1.Rows[e.RowIndex].FindControl("aaaaaa1");