How can I apply a custom sort for a WPF DataGrid from a style?

Question

I have a style that I want to apply to a DataGrid. The DataGrid needs to run my custom sort code instead of the default DataGrid sort. The solution that I tried was as follows:

<Style TargetType="{x:Type DataGrid}" x:Key="FilteredDataGrid">
    <EventSetter Event="Sorting" Handler="DataGrid_Sorting"/>
</Style>

And in the code-behind:

private void DataGrid_Sorting(object sender, DataGridSortingEventArgs e) {

    e.Handled = true;

    //Here is where I put the code for my custom sort.

}//DataGrid_Sorting

However, this code doesn't build. It seems to me that because the DataGrid.Sorting event is not a RoutedEvent, it can't be used in an EventSetter.

How can I customize the sorting for any DataGrid that has my style applied?

Answer 1

There is a workaround to provide a routed event when you only have a "normal" event:

Create an attached property that controls the event forwarding and an attached event that shall replace the original event. In order to do this, create a class DataGridEx (whatever class name you prefer) as a container for the attached property (DataGridEx.EnableSortingEvent) and event (DataGridEx.Sorting).

Also, create a custom RoutedEventArgs class that forwards the original sorting event args

public class DataGridExSortingEventArgs : RoutedEventArgs
{
    public DataGridExSortingEventArgs(RoutedEvent routedEvent, DataGridSortingEventArgs sourceEventArgs) : base(routedEvent)
    {
        SourceEventArgs = sourceEventArgs;
    }

    public DataGridSortingEventArgs SourceEventArgs { get; set; }
}

public static class DataGridEx
{
    public static bool GetEnableSortingEvent(DependencyObject obj)
    {
        return (bool)obj.GetValue(EnableSortingEventProperty);
    }

    public static void SetEnableSortingEvent(DependencyObject obj, bool value)
    {
        obj.SetValue(EnableSortingEventProperty, value);
    }

    // Setting this property to true enables the event forwarding from the DataGrid.Sorting event to the DataGridEx.Sorting RoutedEvent
    public static readonly DependencyProperty EnableSortingEventProperty = DependencyProperty.RegisterAttached(
        "EnableSortingEvent",
        typeof(bool),
        typeof(DataGridEx),
        new FrameworkPropertyMetadata(false, new PropertyChangedCallback(OnEnableSortingChanged)));

    private static void OnEnableSortingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is DataGrid dg)
        {
            if ((bool)e.NewValue)
                dg.Sorting += Dg_Sorting;
            else
                dg.Sorting -= Dg_Sorting;
        }
    }

    // When DataGrid.Sorting is called and DataGridEx.EnableSortingEvent is true, raise the DataGridEx.Sorting event
    private static void Dg_Sorting(object sender, DataGridSortingEventArgs e)
    {
        if (sender is DataGrid dg && GetEnableSortingEvent(dg))
        {
            dg.RaiseEvent(new DataGridExSortingEventArgs(SortingEvent, e));
        }
    }

    // When DataGridEx.EnableSortingEvent is true, the DataGrid.Sorting event will be forwarded to this routed event
    public static readonly RoutedEvent SortingEvent = EventManager.RegisterRoutedEvent(
        "Sorting",
        // only effective on the DataGrid itself
        RoutingStrategy.Direct,
        typeof(RoutedEventHandler),
        typeof(DataGridEx));

    public static void AddSortingHandler(DependencyObject d, RoutedEventHandler handler)
    {
        if (d is DataGrid dg)
            dg.AddHandler(SortingEvent, handler);
    }

    public static void RemoveSortingHandler(DependencyObject d, RoutedEventHandler handler)
    {
        if (d is DataGrid dg)
            dg.RemoveHandler(SortingEvent, handler);
    }
}

Now use those in your style (with local being the xmlns for the namespace where DataGridEx is defined):

<Style TargetType="DataGrid">
    <Setter Property="local:DataGridEx.EnableSortingEvent" Value="True"/>
    <EventSetter Event="local:DataGridEx.Sorting" Handler="DataGrid_Sorting"/>
</Style>

The handler

private void DataGrid_Sorting(object sender, RoutedEventArgs e)
{
    if (e is DataGridExSortingEventArgs args)
    {
        // will prevent datagrid default sorting
        args.SourceEventArgs.Handled = true;
    }

    // More stuff
}

I hope this is what you needed. Had to refresh my memory about attached stuff :)