找不到控件'；MainContent_DropDownList1'；在ControlParameter

我正在调试，我得到了上面列出的奇怪错误。这很奇怪，因为我的DropDownlist1。当我早些时候将它设置为等于这个值时，调试器告诉我它不能将它强制转换为字符串。

这是我的C#，问题就在这里。

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
       Gridsource.ConnectionString = "Data Source=CATALOGSERVER;Initial Catalog=UACOrders;Persist Security Info=True;User ID=Catalog;Password=pass";
        Gridsource.SelectCommandType = SqlDataSourceCommandType.StoredProcedure;
        Gridsource.SelectCommand = "GetOrderHistory";
        Gridsource.DataSourceMode = SqlDataSourceMode.DataSet;
        GridView1.DataSource = Gridsource;
        ControlParameter cp = new ControlParameter();
        cp.ControlID = DropDownList1.ClientID;
        cp.Name = "Company";
        cp.PropertyName = "SelectedValue";
        cp.Type = System.TypeCode.String;
        Gridsource.SelectParameters.Add(cp);
        Gridsource.Page = this;
        this.Controls.Add(Gridsource);
        GridView1.DataBind();
        updatepanel1.Update();
 }

以下是asp.net中涉及的一小部分

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<div id="contentarea">     
    <asp:ScriptManager runat="server"></asp:ScriptManager>
    <asp:UpdatePanel runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional" ID="updatepanel1">
        <ContentTemplate>
            <div>
                <asp:Label Text="Company: " runat="server"></asp:Label>
                <asp:DropDownList ID="DropDownList1" runat="server"
                     DataSourceID="company" DataTextField="COMPANY" DataValueField="COMPANY" 
                    OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
                     AutoPostBack="true" >
                </asp:DropDownList>
                <asp:SqlDataSource ID="company"
                     runat="server" ConnectionString="<%$ ConnectionStrings:UACOrdersConnectionString %>"
                     SelectCommand="SELECT [Customer] AS [COMPANY] FROM [Accounts] ORDER BY [Customer]">
                </asp:SqlDataSource>
                <asp:SqlDataSource ID="Gridsource" runat="server"></asp:SqlDataSource>

奇怪的是，我可以在asp.net代码中正常地使用这种东西。但我似乎无法让它在这个特定网页所需的C#代码绑定中工作。所以，我需要再次找出cp.ControlID应该真正等于什么。

特定异常为"用户代码未处理InvalidOperationException。"在ControlParameter"Company"中找不到控件"DropDownList1"。"

VS将异常放在第39行，即

         GridView1.DataBind();

如果你需要看到更多的代码，只需留下评论，我会发布更多。