What is a good strategy to prevent a change on a parent item in a SharePoint 2010 list if a related child item (i.e. the child of the parent) column is set to open?
For example, I don't want to allow the parent item status column to be set to closed if there are still open child items.
I have written some code that checks if there are open child items but I still cannot show an error message to the end-user and I still cannot prevent the update from being committed. I have tried changing the ItemUpdating to Synchronous execution (from Asynchronous) but it doesn't work.
Currently I can find the open child items but it doesn't seem possible to display an error and stop the execution of the update at this point.