WPF DataGrid CustomSort for each Column

Question

I have a WPF DataGrid bound to a CollectionViewSource that encapsulates an ObservableCollection. This CollectionViewSource has two main objectives:

1) To group each item by a specific property of T. I'm using a ValueConverter in the GroupDescription to get the grouping behaviour I want.

2) To sort the grid by a) primarily the group name (as defined above) and b) the individual group items. I'm achieving this by attaching a custom IComparer to the CollectionViewSource's 'CustomSort' property.

This works great for the most part, however as soon as a column header is clicked, the sorting logic is overridden. I don't want to disable sorting, however I was wondering if it was possible to assign a custom sorting order for a specific column?

To make things a bit clearer, suppose a user clicks 'ColumnA' - at the moment, the sorting logic encapsulated by my CustomSorter is overridden and the DataGrid is now sorted by that property. Rather than sorting by the selected property, I'd like to instead reverse the logic of the CustomSorter.

Answer 1

I created a couple of attached properties which handle this issue. I hope this comes in handy for someone!

First - a simple interface for your directionalised comparer. This extends IComparer but gives us one more property (SortDirection). Your implementation should use this to determine the correct ordering of elements (which would otherwise have been lost).

public interface ICustomSorter : IComparer
{
    ListSortDirection SortDirection { get; set; }
}

Next is the attached behavior - this does two things: 1) tells the grid to use custom sort logic (AllowCustomSort=true) and b) gives us the ability to set this logic at a per-column level.

public class CustomSortBehaviour
{
    public static readonly DependencyProperty CustomSorterProperty =
        DependencyProperty.RegisterAttached("CustomSorter", typeof(ICustomSorter), typeof(CustomSortBehaviour));

    public static ICustomSorter GetCustomSorter(DataGridColumn gridColumn)
    {
        return (ICustomSorter)gridColumn.GetValue(CustomSorterProperty);
    }

    public static void SetCustomSorter(DataGridColumn gridColumn, ICustomSorter value)
    {
        gridColumn.SetValue(CustomSorterProperty, value);
    }

    public static readonly DependencyProperty AllowCustomSortProperty =
        DependencyProperty.RegisterAttached("AllowCustomSort", typeof(bool),
        typeof(CustomSortBehaviour), new UIPropertyMetadata(false, OnAllowCustomSortChanged));

    public static bool GetAllowCustomSort(DataGrid grid)
    {
        return (bool)grid.GetValue(AllowCustomSortProperty);
    }

    public static void SetAllowCustomSort(DataGrid grid, bool value)
    {
        grid.SetValue(AllowCustomSortProperty, value);
    }

    private static void OnAllowCustomSortChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var existing = d as DataGrid;
        if (existing == null) return;

        var oldAllow = (bool)e.OldValue;
        var newAllow = (bool)e.NewValue;

        if (!oldAllow && newAllow)
        {
            existing.Sorting += HandleCustomSorting;
        }
        else
        {
            existing.Sorting -= HandleCustomSorting;
        }
    }

    private static void HandleCustomSorting(object sender, DataGridSortingEventArgs e)
    {
        var dataGrid = sender as DataGrid;
        if (dataGrid == null || !GetAllowCustomSort(dataGrid)) return;

        var listColView = dataGrid.ItemsSource as ListCollectionView;
        if (listColView == null)
            throw new Exception("The DataGrid's ItemsSource property must be of type, ListCollectionView");

        // Sanity check
        var sorter = GetCustomSorter(e.Column);
        if (sorter == null) return;

        // The guts.
        e.Handled = true;

        var direction = (e.Column.SortDirection != ListSortDirection.Ascending)
                            ? ListSortDirection.Ascending
                            : ListSortDirection.Descending;

        e.Column.SortDirection = sorter.SortDirection = direction;
        listColView.CustomSort = sorter;
    }
}

To use it, implement an ICustomComparer (with a parameterless constructor) and in your XAML:

<UserControl.Resources>
    <converters:MyComparer x:Key="MyComparer"/>
    <!-- add more if you need them -->
</UserControl.Resources>
<DataGrid behaviours:CustomSortBehaviour.AllowCustomSort="True" ItemsSource="{Binding MyListCollectionView}">
    <DataGrid.Columns>
        <DataGridTextColumn Header="Test" Binding="{Binding MyValue}" behaviours:CustomSortBehaviour.CustomSorter="{StaticResource MyComparer}" />
    </DataGrid.Columns>
</DataGrid>

Answer 2

The answer given by trilson86 is excellent. However, the third parameter in the two DependencyProperty declarations is incorrect. Instead of DataGrid and DataGridColumn, they should be CustomSortBehaviour, as such:

public static readonly DependencyProperty AllowCustomSortProperty =
        DependencyProperty.RegisterAttached("AllowCustomSort", 
        typeof(bool),
        typeof(CustomSortBehaviour), // <- Here
        new UIPropertyMetadata(false, OnAllowCustomSortChanged));

    public static readonly DependencyProperty CustomSorterProperty =
        DependencyProperty.RegisterAttached("CustomSorter", 
        typeof(ICustomSorter), 
        typeof(CustomSortBehaviour));  // <- Here

I kept getting a warning that the AllowCustomSort property was already registered. A little research led me to the answer here.

At any rate, it's an excellent answer, so thank you.

Answer 3

This answer is very similar to trilson86's solution -- it was based on it -- but it accounts for SortMemberPath in a manner such that the values passed to your comparer are the actual values of the column, rather than the rows. This facilitates far greater re-use on your sorters. Furthermore, it eliminates the need for a custom sort interface altogether.

DataGridSortBehavior.cs

using System;
using System.Collections;
using System.ComponentModel;
using System.Reflection;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

namespace YourNamespace
{
    public class DataGridSortBehavior 
    {
        public static IComparer GetSorter(DataGridColumn column)
        {
            return (IComparer)column.GetValue(SorterProperty);
        }

        public static void SetSorter(DataGridColumn column, IComparer value)
        {
            column.SetValue(SorterProperty, value);
        }

        public static bool GetAllowCustomSort(DataGrid grid)
        {
            return (bool)grid.GetValue(AllowCustomSortProperty);
        }

        public static void SetAllowCustomSort(DataGrid grid, bool value)
        {
            grid.SetValue(AllowCustomSortProperty, value);
        }

        public static readonly DependencyProperty SorterProperty = DependencyProperty.RegisterAttached("Sorter", typeof(IComparer), 
            typeof(DataGridSortBehavior));
        public static readonly DependencyProperty AllowCustomSortProperty = DependencyProperty.RegisterAttached("AllowCustomSort", typeof(bool), 
            typeof(DataGridSortBehavior), new UIPropertyMetadata(false, OnAllowCustomSortChanged));

        private static void OnAllowCustomSortChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            var grid = (DataGrid)obj;

            bool oldAllow = (bool)e.OldValue;
            bool newAllow = (bool)e.NewValue;

            if (!oldAllow && newAllow)
            {
                grid.Sorting += HandleCustomSorting;
            }
            else
            {
                grid.Sorting -= HandleCustomSorting;
            }
        }

        public static bool ApplySort(DataGrid grid, DataGridColumn column)
        {
            IComparer sorter = GetSorter(column);
            if (sorter == null)
            {
                return false;
            }

            var listCollectionView = CollectionViewSource.GetDefaultView(grid.ItemsSource) as ListCollectionView;
            if (listCollectionView == null)
            {
                throw new Exception("The ICollectionView associated with the DataGrid must be of type, ListCollectionView");
            }

            listCollectionView.CustomSort = new DataGridSortComparer(sorter, column.SortDirection ?? ListSortDirection.Ascending, column.SortMemberPath);
            return true;
        }

        private static void HandleCustomSorting(object sender, DataGridSortingEventArgs e)
        {
            IComparer sorter = GetSorter(e.Column);
            if (sorter == null)
            {
                return;
            }

            var grid = (DataGrid)sender;
            e.Column.SortDirection = e.Column.SortDirection == ListSortDirection.Ascending ? ListSortDirection.Descending : ListSortDirection.Ascending;
            if (ApplySort(grid, e.Column))
            {
                e.Handled = true;
            }
        }

        private class DataGridSortComparer : IComparer
        {
            private IComparer comparer;
            private ListSortDirection sortDirection;
            private string propertyName;
            private PropertyInfo property;

            public DataGridSortComparer(IComparer comparer, ListSortDirection sortDirection, string propertyName)
            {
                this.comparer = comparer;
                this.sortDirection = sortDirection;
                this.propertyName = propertyName;
            }

            public int Compare(object x, object y)
            {
                PropertyInfo property = this.property ?? (this.property = x.GetType().GetProperty(propertyName));
                object value1 = property.GetValue(x);
                object value2 = property.GetValue(y);

                int result = comparer.Compare(value1, value2);
                if (sortDirection == ListSortDirection.Descending)
                {
                    result = -result;
                }
                return result;
            }
        }
    }
}

Your Xaml

This should look similar to trilson86's solution as well:

<UserControl.Resources>
    <converters:MyComparer x:Key="MyComparer"/>
</UserControl.Resources>
<DataGrid behaviours:DataGridSortBehavior.AllowCustomSort="True" ItemsSource="{Binding MyListCollectionView}">
    <DataGrid.Columns>
        <DataGridTextColumn Header="Test" Binding="{Binding MyValue}" behaviours:DataGridSortBehavior.Sorter="{StaticResource MyComparer}" />
    </DataGrid.Columns>
</DataGrid>