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

Answer 1

Here is one way:

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

public static class DataGridSort
{
    public static readonly DependencyProperty ComparerProperty = DependencyProperty.RegisterAttached(
        "Comparer",
        typeof(IComparer),
        typeof(DataGridSort),
        new PropertyMetadata(
            default(IComparer),
            OnComparerChanged));

    private static readonly DependencyProperty ColumnComparerProperty = DependencyProperty.RegisterAttached(
        "ColumnComparer",
        typeof(ColumnComparer),
        typeof(DataGridSort),
        new PropertyMetadata(default(ColumnComparer)));

    private static readonly DependencyProperty PreviousComparerProperty = DependencyProperty.RegisterAttached(
        "PreviousComparer",
        typeof(IComparer),
        typeof(DataGridSort),
        new PropertyMetadata(default(IComparer)));

    public static readonly DependencyProperty UseCustomSortProperty = DependencyProperty.RegisterAttached(
        "UseCustomSort",
        typeof(bool),
        typeof(DataGridSort),
        new PropertyMetadata(default(bool), OnUseCustomSortChanged));

    public static void SetComparer(DataGridColumn element, IComparer value)
    {
        element.SetValue(ComparerProperty, value);
    }

    public static IComparer GetComparer(DataGridColumn element)
    {
        return (IComparer)element.GetValue(ComparerProperty);
    }

    public static void SetUseCustomSort(DependencyObject element, bool value)
    {
        element.SetValue(UseCustomSortProperty, value);
    }

    public static bool GetUseCustomSort(DependencyObject element)
    {
        return (bool)element.GetValue(UseCustomSortProperty);
    }

    private static void OnComparerChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var column = (DataGridColumn)d;
        var columnComparer = new ColumnComparer((IComparer)e.NewValue, column);
        column.SetValue(ColumnComparerProperty, columnComparer);
    }

    private static void OnUseCustomSortChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var dataGrid = (DataGrid)d;
        if ((bool)e.NewValue)
        {
            WeakEventManager.AddHandler(dataGrid, nameof(dataGrid.Sorting), OnDataGridSorting);
        }
        else
        {
            WeakEventManager.RemoveHandler(dataGrid, nameof(dataGrid.Sorting), OnDataGridSorting);
        }
    }

    private static void OnDataGridSorting(object sender, DataGridSortingEventArgs e)
    {
        var column = e.Column;
        var columnComparer = (ColumnComparer)column.GetValue(ColumnComparerProperty);
        var dataGrid = (DataGrid)sender;
        var view = CollectionViewSource.GetDefaultView(dataGrid.ItemsSource) as ListCollectionView;
        if (view == null)
        {
            return;
        }
        if (columnComparer == null)
        {
            view.CustomSort = (IComparer)dataGrid.GetValue(PreviousComparerProperty);
        }
        else
        {
            if (!(view.CustomSort is ColumnComparer))
            {
                dataGrid.SetValue(PreviousComparerProperty, view.CustomSort);
            }

            switch (column.SortDirection)
            {
                case ListSortDirection.Ascending:
                    column.SortDirection = ListSortDirection.Descending;
                    view.CustomSort = columnComparer.Descending;
                    break;
                case null:
                case ListSortDirection.Descending:
                    column.SortDirection = ListSortDirection.Ascending;
                    view.CustomSort = columnComparer.Ascending;
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }

            e.Handled = true;
        }
    }

    private class ColumnComparer : IComparer
    {
        private readonly IComparer valueComparer;
        private readonly DataGridColumn column;
        private readonly InvertedComparer inverted;

        public ColumnComparer(IComparer valueComparer, DataGridColumn column)
        {
            this.valueComparer = valueComparer;
            this.column = column;
            inverted = new InvertedComparer(this);
        }

        public IComparer Ascending => this;

        public IComparer Descending => inverted;

        int IComparer.Compare(object x, object y)
        {
            if (x == y)
            {
                return 0;
            }

            if (x == null)
            {
                return -1;
            }

            if (y == null)
            {
                return 1;
            }

            // this can perhaps be a bit slow
            // Not adding caching yet.
            var xProp = x.GetType().GetProperty(column.SortMemberPath);
            var xValue = xProp.GetValue(x);
            var yProp = x.GetType().GetProperty(column.SortMemberPath);
            var yValue = yProp.GetValue(y);
            return valueComparer.Compare(xValue, yValue);
        }

        private class InvertedComparer : IComparer
        {
            private readonly IComparer comparer;

            public InvertedComparer(IComparer comparer)
            {
                this.comparer = comparer;
            }

            public int Compare(object x, object y)
            {
                return comparer.Compare(y, x);
            }
        }
    }
}

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Usage: