PropertyGroupDescription CustomSort is missing the ItemCount in item X - wpf

Question

Preamble:
First I have searched stackoverflow and all of the topics pertaining to sorting grouped datagrids do not apply to this question. In fact none of the answers actually show how to sort by group count without using 3rd partly libraries.
The Problem:
I am trying to sort my datagrid groups by count by overriding the CustomSort property of the PropertyGroupDescription. When I assign a CustomSort method to the GroupDescription, the Compare function's object x CollectionViewGroup always has an ItemCount == 0.
Here is my sample xaml which is mostly taken from Microsoft's help:
<Window
x:Class="GroupedSorting.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:GroupedSorting"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<DataGrid
Name="dg"
Grid.Row="0"
ItemsSource="{Binding ItemVMs}">
<DataGrid.GroupStyle>
<!-- Style for groups at top level. -->
<GroupStyle>
<GroupStyle.ContainerStyle>
<Style TargetType="{x:Type GroupItem}">
<Setter Property="Margin" Value="0,0,0,5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander IsExpanded="True" Background="#FF112255" BorderBrush="#FF002255" Foreground="#FFEEEEEE" BorderThickness="1,1,1,5">
<Expander.Header>
<DockPanel>
<TextBlock FontWeight="Bold" Text="{Binding Path=Name}" Margin="5,0,0,0" Width="100"/>
<TextBlock FontWeight="Bold" Text="{Binding Path=ItemCount}"/>
</DockPanel>
</Expander.Header>
<Expander.Content>
<ItemsPresenter />
</Expander.Content>
</Expander>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</GroupStyle.ContainerStyle>
</GroupStyle>
<!-- Style for groups under the top level. -->
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<DockPanel Background="LightBlue">
<TextBlock Text="{Binding Path=Name, Converter={StaticResource completeConverter}}" Foreground="Blue" Margin="30,0,0,0" Width="100"/>
<TextBlock Text="{Binding Path=ItemCount}" Foreground="Blue"/>
</DockPanel>
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
</DataGrid.GroupStyle>
<DataGrid.RowStyle>
<Style TargetType="DataGridRow">
<Setter Property="Foreground" Value="Black" />
<Setter Property="Background" Value="White" />
</Style>
</DataGrid.RowStyle>
</DataGrid>
</Grid>
</Window>
And here is the code behind:
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Windows;
using System.Windows.Data;
namespace GroupedSorting
{
public partial class MainWindow : Window
{
public ObservableCollection<MyViewModel> ItemVMs { get; set; } = new ObservableCollection<MyViewModel>();
public MainWindow()
{
InitializeComponent();
DataContext = this;
var r = new Random();
for (int i = 0; i < 10; i++)
{
ItemVMs.Add(new MyViewModel()
{
Name = "Group 1",
MyIndex = r.Next()
}); ;
}
for (int i = 0; i < 5; i++)
{
ItemVMs.Add(new MyViewModel()
{
Name = "Group 2",
MyIndex = r.Next()
});
}
for (int i = 0; i < 1; i++)
{
ItemVMs.Add(new MyViewModel()
{
Name = "Group 3",
MyIndex = r.Next()
});
}
List<MyViewModel> sortedItems = new List<MyViewModel>();
var groups = ItemVMs.GroupBy(x => x.Name);
foreach (var group in groups.OrderByDescending(x => x.Count()))
{
sortedItems.AddRange(group);
}
ItemVMs.Clear();
ItemVMs = new ObservableCollection<MyViewModel>(sortedItems);
Loaded += MainWindow_Loaded;
}
private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
var sourceView = CollectionViewSource.GetDefaultView(dg.ItemsSource) as ListCollectionView;
var gd = new PropertyGroupDescription(nameof(MyViewModel.Name));
gd.CustomSort = new GroupComparer();
sourceView.GroupDescriptions.Add(gd);
sourceView.Refresh();
}
}
public class MyViewModel
{
public string Name { get; set; }
public int MyIndex { get; set; }
}
public class GroupComparer : System.Collections.IComparer
{
public int Compare(object x, object y)
{
if (!(x is CollectionViewGroup xViewGroup))
return 0;
if (!(y is CollectionViewGroup yViewGroup))
return 0;
Debug.WriteLine($"{xViewGroup.Name} {xViewGroup.ItemCount}, {yViewGroup.Name} {yViewGroup.ItemCount}");
if (xViewGroup.ItemCount < yViewGroup.ItemCount)
return 1;
else if (xViewGroup.ItemCount > yViewGroup.ItemCount)
return -1;
return 0;
}
}
}
When the code is run, the xViewGroup.ItemCount is always equal to 0 causing the sort method to fail.

Answer 1

Cast the ICollectionView returned from CollectionViewSource.GetDefaultView to a ListCollectionView and set the CustomSort property of this one to your custom IComparer:
private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
var sourceView = CollectionViewSource.GetDefaultView(dg.ItemsSource) as ListCollectionView;
var gd = new PropertyGroupDescription(nameof(MyViewModel.Name));
sourceView.CustomSort = new GroupComparer();
sourceView.GroupDescriptions.Add(gd);
sourceView.Refresh();
}