Cast a HtmlGenericControl as a DropDown

Question

I have quite complicated set of HTML that I want to trawl looking for inputs that match various criteria. I hope to use something along the lines of:

private void setup()
{
    masterContainer.InnerHtml = @"
    <div>crazy
        <div>unknown
            <div>html layout
                <select id='crazySelectIdentifier_id1' runat='server'>
                    <option value='1'>Item1</option>
                    <option value='2'>Item2</option>
                </select>
            </div>
        </div>
    </div>
    <div>
        <div>
            <select id='crazySelectIdentifier_id2' runat='server'>
                <option value='1'>Item1</option>
                <option value='2'>Item2</option>
            </select>
        </div>
    </div>
    <div>
    </div>";
}

private void recursiveTrawl(HtmlGenericControl currentOuterControl)
{                            
    for (int i = 0; i < currentOuterControl.Controls.Count; i++)
    {
        HtmlGenericControl currentControl = (HtmlGenericControl) currentOuterControl.Controls[i];
        if(currentControl.HasControls())
        {
            recursiveTrawl(currentControl);
        }
        else
        {
             String[] controlArr = currentControl.ID.ToString().Split('_');
             String currentId = controlArr[1];

             if (currentId.Equals("somethingspecific"))
             {
                  //THE PROBLEM IS HERE
                  DropDownList dropdown = (DropDownList)currentControl;

However, I get the error- Cannot convert type 'System.Web.UI.HtmlControls.HtmlGenericControl' to 'System.Web.UI.WebControls.DropDownList'

I've tried using HtmlSelect as well with a similiar error. I just need to know how I can get access to the selected values in the drop down lists I'm interested in.

Thanks in advance.

Answer 1

Try this:

WebForm

<asp:PlaceHolder ID="PlaceHolder1" runat="server" />

Extension Method

public static class ControlExtensions
{

    public static void FindControlByType<TControl>(this Control container, ref List<TControl> controls) where TControl : Control
    {
        if (container == null)
            throw new ArgumentNullException("container");

        if (controls == null)
            throw new ArgumentNullException("controls");

        foreach (Control ctl in container.Controls)
        {
            if (ctl is TControl)
                controls.Add((TControl)ctl);

            ctl.FindControlByType<TControl>(ref controls);
        }
    }

}

Code

string html = @"<div>
                    <select id='Sel1' runat='server'>
                        <option value='1'>Item1</option>
                        <option value='2'>Item2</option>
                        <option value='3'>Item3</option>
                    </select>
                </div>
                <div>
                    <select id='Sel2' runat='server'>
                        <option value='4'>Item4</option>
                        <option value='5'>Item5</option>
                        <option value='6'>Item6</option>
                    </select>
                </div>";

Control ctl = TemplateControl.ParseControl(html);
PlaceHolder1.Controls.Add(ctl);

List<HtmlSelect> controls = new List<HtmlSelect>();
PlaceHolder1.FindControlByType<HtmlSelect>(ref controls);

foreach (HtmlSelect select in controls)
{
}

Answer 2

This cast will always error at compile time because there's no inheritance relationship between HtmlGenericControl and HtmlSelect. An object cannot be both. Once an object has been successfully cast as HtmlGenericControl (as the function argument is), the compiler knows for certain that it can't also be a HtmlSelect, so it won't let you even try the cast.

Even if the compile worked, you would get an error at runtime, because the <select> is not a HtmlGenericControl.

Your solution is not to bother casting anything to HtmlGenericControl. Just use the Control class, as it comes out of the Controls collection. The only cast you should do is to HtmlSelect, when you know you're looking at the right object.