@Nathan: This error most often happens when there is a discrepancy between the Telerik.Web.UI
assembly (the assembly, containing the runtime definitions of the controls) and its Telerik.Web.Design
counterpart (the assembly, containing the Visual Studio design time definitions).
Can you please, check if there is a Telerik.Web.Design
assembly with version, corresponding to the Telerik.Web.UI
one in the project's BIN exists in the GAC? Can you ensure you're running Visual Studio 2008 SP1?
@Doug, can you observe if the error happens after performing a drag-and-drop of a control from the Visual Studio Toolbox? It could be that the ToolBox is populated with controls from a Telerik.Web.UI
assembly, having a different version than the one in the project's BIN. Thus, when a drag-and-drop is performed, the IDE loads the assembly twice in its AppDomain, causing the problem. A consequent close and start of the IDE causes a new, clean AppDomain to be created, which solves the problem.
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