Hi Frank Filippis,
I have been testing the problem on both 32-bit and 64-bit machines and here's what I found:
- On 32-bit Vista machines you can't drop a control from the toolbox at all -- you get an "Invalid FORMATETC" error message.
- On 64-bit Vista you can drop the control, and it works, but you get a squiggly line below your closing tag and a tooltip that says AG_E_UNKNOWN_ERROR.
I removed the toolbox tab and added the controls to the toolbox myself. Both problems disappeared. My bet is that this is a problem with the Silverlight support in the current version of VS2008. There is no documented way to add controls to the SL toolbox, and our installer tries to add the controls in a manner similar to the way prescribed for WPF, WinForms, ASP.NET, etc. That undocumented way to add controls to the toolbox seems to have its issues like the two listed above.
We are currently looking for ways to work around the problems. If we do not find a proper way to register controls with the toolbox we may be disabling automatic registration entirely. It may be better to require customers to add controls to the toolbox by hand than require them to deal with cryptic error messages.
For now, just remove the toolbox tab generated by the RadControls for Silverlight installer and add the controls to the toolbox yourself. Make sure you restart Visual Studio, as it sometimes caches toolbox items and assembly references.
the Telerik team