"Cannot have multiple items selected in a dropdownlist" Error in AppCode

Question

I have a user defined type dropdownlist(DDL) on my web form page which is inherited from System.Web.UI.WebControls.DropDownList. I fill my DDL in Page's code file (.aspx.cs) and AppCode like this without getting error.

public static void FillDropDownList(ListControl drp)
{
    drp.Items.Clear();
    drp.Items.Add(new ListItem("Please select", ""));
    drp.Items.Add(new ListItem("False", false.ToString()));
    drp.Items.Add(new ListItem("True", true.ToString()));
}

Then I am initializing my DDL:

drp.SelectedValue = "False";

Initialization works when Fill method in Page's Code File (aspx.cs). Initialization doesn't work when Fill method in AppCode and I am getting the following error.

Cannot have multiple items selected in a DropDownList. Please send a copy of this message to the Support. 

at System.Web.UI.WebControls.DropDownList.VerifyMultiSelect() 
at System.Web.UI.WebControls.ListControl.RenderContents(HtmlTextWriter writer) 
at System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.Control.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) 
at System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at ASP.fleetnetnew_masterpages_fleetpopuppage_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\fleetintlpdvl\204737b8\cd934b62\App_Web_jw12vu5i.2.cs:line 0 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.Control.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) 
at System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) 
at System.Web.UI.Page.Render(HtmlTextWriter writer) 
at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) 
at System.Web.UI.Control.RenderControl(HtmlTextWriter writer) 
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I need to use AppCode for filling DDLs so I can Fill my DDLs from a central point. What am I missing here?

Answer 1

1) Add the drop down list within your aspx page (use AppendDataBoundItems)

<asp:DropDownList runat="server" ID="drpTest" AppendDataBoundItems="true" />

2) Within your c# code behind page

    public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            FillDropDownList(drpTest);

            // select False after filling drop down list
            drpTest.Items.FindByValue("False").Selected = true;
        }

        public static void FillDropDownList(ListControl drp)
        {
            drp.Items.Clear();
            drp.Items.Add(new ListItem("Please select", ""));
            drp.Items.Add(new ListItem("False", false.ToString()));
            drp.Items.Add(new ListItem("True", true.ToString()));
        }
    }