Hi,

I'm a french absolute beginner in both StrataFrame and VB.Net (but 23 years with Clipper and Fox)...

Data are on several databases on SQL Server (DataBcuv1, DataBcuv2, ...) and global parameters are in ParaBcuv on the same Server. The most important of these is the correspondence between "folder" ID and database name (first folder may be linked to databcuv4, if my customer requires it).

In my main form, I launch (in its shown method) a form "choixdossier" as showdialog(), to chooses working folder (i.e. database), and I return a connect string in a property of my main form.

I use one BO for this first connection to parabcuv, and I need to dynamically re-set the connection strings of all BO to databcuv which are used in all forms in that application (all BO are in a BO library referenced in final application).
But method SetDataSources is in appmain.vb, not in my main form :-(

How can I do that?

Yep, Greg is absolutely right (once again ).  If you change the ConnectionString property on a DataSource, you're not guaranteed that it's going to be updated throughout the application (it gets cached off for speed so it doesn't need to be looked up every time).  So, instead, remove the DataSource and re-add it to the DataSources collection, just like in Greg's sample, and all of the business objects will update and start using the new data source.