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Design-Time Errors Occur on a Page Using Telerik Controls


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When working with Telerik UI for ASP.NET AJAX or dragging a Telerik control from the ToolБox to the design surface, design-time errors occur on a page that uses Telerik controls.


Depending on the use case, the possible causes for such errors may be the following:

  • If the Telerik.Web.UI.dll assembly is updated while a page using Telerik controls is in Design-time, Visual Studio creates two different versions of the assembly. Hence, two different versions of the same Telerik controls are available.

  • If you try to drag a Telerik control from the Toolbox to the design surface and get an error, the possible reasons are various. Refer to the resource links below.

  • After upgrading Telerik UI for ASP.NET AJAX to Q1 2009, Visual Studio 2008 has a problem with loading GAC-ed design-time assemblies of control libraries deployed in the Bin folder of a website.

Error Messages

  • (Two assembly versions) Error Creating Control - Telerik control 'Property' cannot be initialized. Details: Unable to cast object of type 'Type' to type 'Type'

  • (Dragging a Telerik control from the ToolBox) Error creating control. Unable to cast object of type 'Telerik.Web.UI.Radxxx' to type 'Telerik.Web.UI.ControlItemContainer

  • (Problem loading GAC-ed assemblies) Error creating control - ControlName or Failed to create designer "Telerik.Web.UI.ControlName..."


Depending on the possible cause for this issue, use the following approaches to handle it:

  • (Two assembly versions) Restart Visual Studio.

  • (Dragging a Telerik control from the ToolBox) Refer to the following resources:

  • (Problem loading GAC-ed assemblies) To solve this issue, use any of the following approaches:

    • To Bin-deploy the design-time assembly, add a reference to Telerik.Web.Design.dll to your website or application, remove Telerik.Web.Design.dll from the GAC, and restart Visual Studio.

    • To GAC-deploy the control library assembly, add Telerik.Web.UI.dll in the GAC, remove Telerik.Web.UI.dll from your Bin folder, and restart Visual Studio.

    • Copy Telerik.Web.UI.dll to C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\PublicAssemblies and restart Visual Studio.

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