Unrecognized tag prefix or device filter 'asp' in VS2012

unrecognized tag prefix or device filter 'webopt'
unrecognized tag prefix or device filter 'telerik'
unrecognized tag prefix or device filter 'dx'
unrecognized tag prefix or device filter 'cc1'
unrecognized tag prefix uc1
asp tag is not recognized

I recently installed VS2012 and working on work projects that I used to work on with VS2010. I never had an issue with VS2010 not recognizing asp prefixed tags but VS2012 is. I tried deleting the ReflectedSchemas folder under

C:\Users[User_name]\AppData\Roaming\Microsoft\VisualStudio\11.0\

but still not working. Anyone ran into this issue in VS2012?

Closing and reopening the file (.aspx file not solution) from solution explorer worked for me in visual studio 2015. Before trying anything else you can give it a try.

Unrecognized Tag Prefix or Device Filter 'asp', In fact, every ASPX control on the page displayed the error, “Unrecognized tag prefix or device filter 'asp.'” What's up with that? Thinking that  Asp.Net, Dot Net 4.0 Example, in the below label control there is asp tag prefix, that is getting underlined in green and shown as "Unrecognized tag prefix or device filter 'asp'" <asp:Label ID="lblErrorInitiatorDetails" runat="server" Visible="true" ForeColor="Red"> 1 solution

Do you happen to have something like this in your Web.Config:

<pages>
  <controls>
    <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
  </controls>
</pages>

In that case, remove the entries for the asp tagPrefix (remove the entire section if no other tagPrefixes are left). This worked for me.

Unrecognized tag prefix or device filter 'asp', When editing HTML markup all <asp: tags are flagged (green line under "asp") with the message: "Unrecognized tag prefix or device filter 'asp'" In fact, every ASPX control on the page displayed the error, “Unrecognized tag prefix or device filter ‘asp.’” What’s up with that? Thinking that there must be something wrong with our development environment, or at least something wrong with how we have the project configured, I started my search on Google.

If you are having this "Unrecognised tag prefix 'asp' in VS2012" problem in a Content Page,

Check whether the Master page, under which you are adding the content page is missing the **<body>** or <form> Tag!

If it is missing this tag by any chance, then this <asp> tag is not recognized.

'asp' is an unrecognized tag prefix or device filter, For some unforeseen reason VS now reports the error -'asp' is an unrecognized tag prefix or device filter on all my .aspx pages. The solution  Solved: “Unrecognized tag prefix or device filter ‘telerik'” Unrecognized tag prefix or device filter ‘telerik’ device filter 'telerik' telerik

For a project setup using .Net 4.0 following lines work.

    <pages>
        <controls>
            <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
            <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
            <add tagPrefix="asp" namespace="System.Web.UI.DataVisualization.Charting" assembly="System.Web.DataVisualization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        </controls>
    </pages>

Unrecognized tag prefix or device filter 'asp', Unrecognized tag prefix or device filter 'asp ..error occur on every source code pageHellp!! Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Learn more Unrecognized tag prefix or device filter 'asp' in VS2012

<%@ Page Title="" Language="VB" MasterPageFile="something" AutoEventWireup="false" CodeFile="something" Inherits="something" %>
<br/><br/>

Please give an 'enter' after first line. it worked for me

Solved: "Unrecognized tag prefix or device filter 'telerik'", Steps to solve Visual Studio error that says "Unrecognized tag prefix or device filter 'telerik'" unrecognized tag prefix or device filter 'telerik'. in UI for ASP.NET AJAX General Discussions - Telerik Forums.

Unrecognized tag prefix or device filter cc1-what it means? Please , Your code. Hide Copy Code. <cc1:calendarextender means that you have a library registered as cc1 and in it is a control named 

Page Layout editing in Visual Studio: "Unrecognized tag prefix , Has 'asp' green-underlined stating "Unrecognized tag prefix or device filter 'asp'". Among other annoyances, there's no intellisense for controls, no auto format of 

Unrecognized Tag Prefix in UI for ASP.NET AJAX General , Mousing over the underlined tags reveals the following error message "​Unrecognized tag prefix or device filter 'telerik'". I've also lost Telerik intellisense.

Comments
  • Can you share some code and example ? What are this tags, maybe the dll that referred on this tag fails to load.
  • Something like: <asp:PlaceHolder ID="test" runat="server"> would not be recognized. This is legit for VS2010
  • Even asp.net controls need to include some headers and some dll files. Check that your files are included correct.
  • Ya the correct DLLs as well as the tag definitions in web.config are there. This works in VS2010.
  • Yes. I am getting this with VS2012 Update 1 in my SharePoint 2010 project on an Application Page. I've tried deleting files and it didn't work. I am lost without Intellisense on this.
  • All these comments are valid but this one worked for me. Worked in VS 2017 even though closing and restarting Visual Studio did not.
  • Lol. When the solution really is the thing a non-developer would've just tried before asking
  • Oh... This worked for me. :)
  • I did have those in my web.config. I removed them but intellisense is still not recognizing the asp tags. This worked for VS2010 even with those entries in the web.config
  • Ok, it worked for me in VS2012 when i removed those tags (altough not in VS2010 anymore, so you need to put them back for that)
  • I actually had a reference to asp and telerik controls. I was getting the same error for telerik controls though. I removed the references, tried to compile (get errors) and then re-added the references and it works now. Must be a bug with VS.
  • Removing <add tagPrefix="asp" namespace="System.Web.UI.DataVisualization.Charting" assembly="System.Web.DataVisualization, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/> has just worked for me. Thanks for the tip.
  • Worked for me too, If someone has this same issue, try apply this change, close and open again Visual Studio.
  • Thank you! Obscure problem but this fixed it for me. I had been writing the <body> tag dynamically, in code, eg. <%= GetBodyTag() %>, so the tag did not appear as such in the .master file's HTML. Changed it to eg. <body class="<%= GetBodyClasses() %>"> - now VS knows the tag is there and the errors have disappeared. Quite ridiculous that writing a tag in code, instead of in HTML, breaks the whole of VS intellisense.