Element is not a known element warning in Visual Studio when using User controls

Question

I have a Visual Studio 2008 project that is showing the following warning when using User Controls, and I haven’t been able to find a solution anywhere.

Answer 1

Check you might be on ContentPlaceHolderID="MainContent" on Asp.net, so place the code on a new placeholder

Answer 2

I found that the issur was resolved when I changed the Build Action from "content" to "compile" for the .ascx file and afterwards chaned it back.

Answer 3

This sounds like a classic re-build your solution and "close and re-open Visual Studio" problem.

It's possible it may also be related to a similar problem I had which I answered at Resolving "Validation (): Element ‘xxxx’ is not supported" warning in Visual Studio 2005/2008.

Answer 4

Apparently this can also happen if the Namespace name in the .ascx file doesn't match the namespace in the ascx.cs (codebehind) file. Just one more issue to check.

Answer 5

This can also occur if the element you're trying to add is within the tags of another element that it shouldn't be within.

For Example:

<asp:Button ID="button" runat="server" >
    <asp:Repeater ID="repeater" runat="server"></asp:Repeater>
</asp:Button>

Or in my case, placing an <asp:Repeater> in an <asp:UpdatePanel> and forgetting to put it in the <ContentTemplate>:

<asp:UpdatePanel ID="upPanel" runat="server">
    <ContentTemplate>
        <asp:Repeater ID="rep" runat="server">

        </asp:Repeater>
    </ContentTemplate>
</asp:UpdatePanel>

Answer 6

From the OP:

The apparent solution to this is to make sure that the TagName is the name of control class.

So for my example, the following displayed the warning:

<%@ Register Src="~/path/to/Control.ascx" TagName="tagName" TagPrefix="tagprefix" %>

<tagprefix:tagName runat="server" id="controlID" />

But changing it to:

<%@ Register Src="~/path/to/Control.ascx" TagName="Control" TagPrefix="tagprefix" %>

<tagprefix:Control runat="server" id="controlID" />

fixes it.