>I have been having similar problems when displaying my foreign key values in comboboxes.

I think that the problems relate to how these objects are registered with the form (and the binding context).
Must you regester these "lookup" business objects (used by comboboxes and pickers) as ChildObjects?
Would this assist in push error handling by Business Rules for these "lookup" business objects up to a parent object?
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I'm not sure we're having the same problem or not. In my case, the rules being checked are for the primary business object, the lookup is just a lookup. That is, I'm just checking that a FK column in the primary biz object, for example, is not null. For me anyway, there's no need to bubble up any broken rules for child biz objects (no header/detail stuff yet).

Also, the ErrorProvider shows up just fine for a ComboBox and a ListBox (they both display info from the FK), it's just the mmPicker that's giving me trouble. Interstingly enough, if I use the the dialog method to display broken rules/warnings, they show up just fine.

Been going through the source code, but as of yet, I can't determine why the ErrorProvider does not show up in my case. Still looking... I know it's there somewhere!