The QueryGroups event is raised to allow the data items contained in a virtualized collection view to be grouped using the information provided by the QueryGroupsEventsArgs received in the event parameters.

The ChildGroupPropertyName of the QueryGroupsEventArgs represents the name of the item property that should be queried in order to find its distinct values and their count taking into account the parent groups, which can be retrieved through the GroupPath property. The GroupPath property is a generic ReadOnlyCollection of DataGridGroupInfo objects, which each expose a property name and a value representing the parent group path. The order of the DataGridGroupInfo objects in the collection corresponds to their order in the containing virtualized collection view and parent DataGridControl. A DataGridGroupInfo also exposes its matching group description.

Once the distinct values for the queried item property have been retrieved, they must no longer be added to the ChildGroupNameCountPairs collection of the QueryGroupsEventArgs received in the event parameters, but rather passed as a parameter to the QueryGroupsEventArgs.AsyncQueryInfo.EndQuery method. The order in which the GroupNameCountPair objects are added to the collection will correspond to their order in the virtualizing collection view and parent datagrid. The ChildSortDirection property can be consulted to know the order in which the GroupNameCountPair objects should be added.

For a sample on a typical way to handle the QueryGroups event in an SQL-based scenario, take a look at the Data Virtualization sample.