Unknown server tag telerik:grid

When trying to open aspx forms that contained Telerik controls I received the error message, "Unknown server tab telerik:grid". My colleague was able to open thse

We tried cleaning and rebuilding, opening and closing the project, which made no difference.

Then we noticed that in the solution explorer there were warning symbols against the telerik references.

My colleague had referenced the Telerik controls from the bin folder in the solution, but this folder had not been checked into source safe.

When my colleague added the bin folder to source safe, and I rechecked the code out, all worked fine.

So the answer we have found is to make sure that the Telerik dlls are in the project binary folder, and to reference them from there.

A similar problem occurred when the project refused to build. This was solved by adding a blank form, and dragging one of the telerik controls onto it.