Question

I’ve created a UserControl in VB.NET and added it to a form. But it looks like I’ve done it wrong as it is underlined with the message:

ctlDropDown is not a known element. This can occur if
there is a compilation error in the Web site or the web.config file is missing.

When I try to run it I get the following error:

myControl is not allowed here because it does not extend class System.Web.UI.UserControl.

Here’s the code:

control itself…

   <%@ Control Language="vb" AutoEventWireup="false" CodeBehind="ctlDropDown.ascx.vb" Inherits="myProj.UserControls.ctlDropDown" TargetSchema="http://schemas.microsoft.com/intellisense/ie5" %>
  <asp:textbox id="txtValue" style="Z-INDEX: 101; LEFT: -100px; POSITION: absolute; TOP: -200px"
        tabIndex="0" runat="server"></asp:textbox><asp:panel id="pnlMain" style="scrolling: no" Width="100%" runat="server" HorizontalAlign="Right"
        BorderWidth="1px" BorderStyle="Solid">
        <TABLE cellSpacing="0" cellPadding="0" width="100%" border="0">
            <TR>
                <TD vAlign="top" width="100%">
                    <asp:TextBox id="txtData" tabIndex="9500" runat="server" Width="100%" Height="100%" BorderStyle="None"
                        BorderWidth="0px"></asp:TextBox></TD>
                <TD vAlign="top">
                    <asp:ImageButton id="imgDownButton" runat="server" BackColor="ControlLight" BorderStyle="Outset"
                        BorderWidth="2px" ImageUrl="../Arrow.gif"></asp:ImageButton></TD>
            </TR>
        </TABLE>
    </asp:panel><asp:listbox id="lstList" tabIndex="0" Height="205px" Width="100%" runat="server" Visible="False"></asp:listbox><asp:textbox id="txtActiveElementClientID" style="Z-INDEX: 100; LEFT: -100px; POSITION: absolute; TOP: -200px"
        tabIndex="0" runat="server"></asp:textbox>

on the form…

<%@ Register TagPrefix="uc1" TagName="ctlDropdown" Src="../UserControls/ctlDropDown.ascx" %>

and…

<table>
<tr>
<TD style="POSITION: absolute;">
     <uc1:ctlDropDown ID="ctlDropDown1" runat="server"></uc1:ctlDropDown>
</TD>
</tr>
</table>

Answer 1

You’ve shown us the markup for your user control. However, there should also be a file which has the actual class for that object. Your markup references it here:

CodeBehind="ctlDropDown.ascx.vb"

This file should contain a class, referenced in your markup here:

Inherits="myProj.UserControls.ctlDropDown"

This class should extend (inherit from) the class System.Web.UI.UserControl. Something like:

Namespace myProj.UserControls
  Public Class ctlDropDown Inherits System.Web.UI.UserControl

    ' Your class implementation.  Likely a Page_Load method perhaps.

  End Class
End Namespace

(Note that the class may be marked as a “partial” class, depending on how the user control was created.)

This is needed by the compiler more so than the markup. The actual controls, user controls, etc. throughout an ASP .NET web application all boil down to classes interacting with each other. Without a class backing the markup, it’s not known to the runtime and can’t be used.