Solved: “Unknown server tag telerik:RadScriptManager”

If you are using third-party .NET component libraries such as Telerik or DevExpress, you may come across the error message below while trying to include controls in your page.

Server Error in ‘/TestASPNET’ Application.

Parser Error
Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Unknown server tag ‘telerik:RadScriptManager’.


Make sure to include 2 DLL files below in your project references of Visual Studio:

  • Telerik.Web.UI
  • Telerik.Web.UI.Skins

If you are still receiving the error above, add a reference to your Telerik assembly in your web.config file. You can use the following lines to make this reference.

  <pages viewStateEncryptionMode="Always" controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID">
      <add tagPrefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI"/>

Solved: “Unrecognized tag prefix or device filter ‘telerik'”

I came across “unrecognized tag prefix” error while trying to use Telerik’s ASP.NET AJAX controls. Here is how the error message looks like:

Warning – Unrecognized tag prefix or device filter ‘telerik’


  1. Make sure to add Telerik DLLs into the bin folder of your project
  2. Add the following handlers into your web.config file (in section):

            <add tagPrefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI"/>
          <add path="ChartImage.axd" type="Telerik.Web.UI.ChartHttpHandler" verb="*" validate="false"/>
          <add path="Telerik.Web.UI.SpellCheckHandler.axd" type="Telerik.Web.UI.SpellCheckHandler" verb="*" validate="false"/>
          <add path="Telerik.Web.UI.DialogHandler.aspx" type="Telerik.Web.UI.DialogHandler" verb="*" validate="false"/>
          <add path="Telerik.RadUploadProgressHandler.ashx" type="Telerik.Web.UI.RadUploadProgressHandler" verb="*" validate="false"/>
          <add path="Telerik.Web.UI.WebResource.axd" type="Telerik.Web.UI.WebResource" verb="*" validate="false"/>

Similar solutions apply to the same type of message for other components.

Solved: Telerik RadGrid doesn’t retain data after a postback

You may come across this issue when you try to sort, filter or group your table that is built with Telerik RadGrid component. Each of these sorting, filtering, and grouping features trigger a postback. You may not see the data in your table after the postback is completed.


Make sure you define OnNeedDataSource attribute for your RadGrid object and handle NeedDataSource event. Telerik uses NeedDataSource event each time RadGrid is need to be bound with a data source.

ASPX file:

<telerik:RadGrid RenderMode="Lightweight" ID="RadGrid1" runat="server" CellSpacing="0"
GridLines="None" OnNeedDataSource="RadGrid1_NeedDataSource1" AllowPaging="true" PageSize="10">
     <MasterTableView AutoGenerateColumns="true" DataKeyNames="CustomerID">

Code-behind file:

protected void RadGrid1_NeedDataSource1(object sender, Telerik.Web.UI.GridNeedDataSourceEventArgs e)
      RadGrid1.DataSource = GetDataTable("SELECT CustomerID, CompanyName, ContactName FROM Customers");