A good deal of yesterday and some of this morning I've been fiddling around with nested ASP.NET DataGrids. Binding nested grids is pretty easy and they show all you want, but editing items in a nested grid just doesn't work as easy as editing in a simple Grid. In fact, it doesn't work at all. What happens is that you can put a nested grid into edit mode, but you never seem to be able to catch any Update/Cancel events from the edited item.
I tried to look for a solution by asking Google, but the answers that I found were very unsatisfactory, since there was no explanation on why exactly it doesn't work. So, here's why ... and it's very, very simple: Nested DataGrids lose all of their ViewState on any roundtrip. That seems to be some sort of problem that's actually related to how the entire TemplateControl infrastructure works, but that's what it is.
Since that's the case, the EditItemIndex isn't preserved across the roundtrip and the DataGrid doesn't know how to dispatch the Update event. Now, how do I work around it? Again, pretty simple: You need to store the EditItemIndex (and SelectedItemIndex, etc.) of the nested data-grid in the Page's ViewState whenever they change (Edit event, Cancel event, etc.), keyed by the UniqueID of the DataGrid and a matching suffix. When you reload the sub-grid on a roundtrip, recover the value(s) from the ViewState of the page and DataBind().
I've put the workaround into my current working copy for dasBlog (the OPML editor gets a hierarchical editor now) and it works great. Next build that gets released, you can look at it.