在代码隐藏中找不到HTML控件

Question

I have a div (id=main), it contains 15 html select controls inside and their IDs are ddl1, ddl2, ddl3, ... and so on.

Now I want to dynamically select or assign their indexes from an XML file, so I wrote this code.

private void readxml(string spath) {
  XmlDocument doc = new XmlDocument();
  doc.Load(spath);
  //doc.LoadXml(spath);
  XmlNodeList xmlnodes = doc.SelectNodes("/Hedge/*");
  for (int i=1; i <= 15; i++) {
    (main.FindControl("ddl" + i) as DropDownList).SelectedIndex = Int32.Parse(xmlnodes[i].InnerText);
  }
}

But an error is occuring here...

Object reference not set to an instance of an object.

It's maybe because it's not able to find the controls (HTML select controls)... Can anyone tell me the reason or solve my problem?

Answer 1

您是否将runat="server"添加到控件和Id="ddl1" ？

Answer 2

In case of such markup:

<div id="main" runat="server">
    <select id="ddl1" runat="server"></select>
</div>

The type of ddl1 won't be DropDownList but rather HtmlSelect, so change your code to:

(main.FindControl("ddl" + i) as HtmlSelect).SelectedIndex = Int32.Parse(xmlnodes[i].InnerText);

If still null error, good chance that xmlnodes is null meaning something went wrong loading the XML document or selecting the nodes.

Answer 3

There are two thing that may have went wrong. One is obvious

  (main.FindControl("ddl" + i) as DropDownList).SelectedIndex = 
            Int32.Parse(xmlnodes[i].InnerText);

Since you are not using asp.net dropdown list so you can not cast it into this. So it should be

     (main.FindControl("ddl" + i) as HtmlSelect).SelectedIndex = 
                                                    Int32.Parse(xmlnodes[i].InnerText);

Another could be the attribute runat="server" which needs to be there if you are suing html controls and want to access it on C# page.

   <select id="Select1" runat="server">
     <option value="1" Selected="True"> Item 1 </option>
     .....
  </select>