Hi Christophe,

Thanks for posting!

For the current issue, based on my experience, this functionality is based
on design time support in ASP.NET custom control. Unfortunately, I don't
find any specific sample codes for the current requirement. However, there
is a good resource from MSDN document explains the whole architecture of
the design time support. The document also demonstrates how to use the
design time support:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/htm
l/cpcondesign-timeforwebforms.asp

If you are interest in the theory of design time support, please look the
following article:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/htm
l/cpconenhancingdesign-timesupport.asp

Although there is no specific sample for the current issue, there still
have many resources from third party to demonstrate how to do:
http://www.codeproject.com/useritems/design-time-integration.asp

http://www.15seconds.com/issue/040422.htm

Hope this will be helpful!

Regards,

Yuan Ren [MSFT]
Microsoft Online Support