WPF - Grouped CollectionView showing multiple times same group - wpf

Question

I'm having a problem with grouping a CollectionView which is bound to an ItemsControl.
In my ViewModel I have multiple groups. A Group is a collection of Person, and each Person knows which group it belongs to.
See Group and Person classes at the end.
- Here's what I want to do : I want to be able to show a collection of Person(s) grouped by their groups. I emphesize that I want to create these groups in the view from a collection of Persons, not from a collection of Groups.
- Here's how I do it :
I have a list which contains every Person across all groups (the list is called "Everyone"), and I have a CollectionView which I create from Everyone called "EveryoneGrouped". I then add a PropertyGroupDescription created from "Group" to my EveryoneGrouped collection.
- Here's my problem :
Groups appear multiple times, in fact groups appear as many times as the amount of Person they contain.
If I have the following groups :
- Group ONE ["Alpha One"]
- Group TWO ["Alpha Two", "Beta Two"]
- Group THREE ["Alpha Three", "Beta Three", "Gamma Three"]
It will produce the following :
Group ONE appears only once since it contains only one Person. I would like for groups TWO and THREE to appear only once as well.
I just can't put my finger on what I'm doing wrong, any help would be greatly appreciated.
Edit : multiple Groups or Persons can have the same name.
Here's my code which produces the previous screenshot :
ViewModel
public class ViewModel
{
public CollectionView EveryoneGrouped { get; private set; }
private List<Person> Everyone { get; set; } = new List<Person>();
private List<Group> AllGroups { get; set; } = new List<Group>();
public ViewModel()
{
populateGroups();
populateEveryoneCollection();
createCollectionView();
}
private void populateGroups()
{
Group one = new Group
{
new Person { PersonName = "Alpha One" }
};
one.GroupName = "ONE";
Group two = new Group
{
new Person { PersonName = "Alpha Two" },
new Person { PersonName = "Beta Two" }
};
two.GroupName = "TWO";
Group three = new Group
{
new Person { PersonName = "Alpha Three" },
new Person { PersonName = "Beta Three" },
new Person { PersonName = "Gamma Three" }
};
three.GroupName = "THREE";
AllGroups.Add(one);
AllGroups.Add(two);
AllGroups.Add(three);
}
private void populateEveryoneCollection()
{
foreach(Group group in AllGroups)
{
foreach(Person person in group)
{
Everyone.Add(person);
}
}
}
private void createCollectionView()
{
EveryoneGrouped = (CollectionView)CollectionViewSource.GetDefaultView(Everyone);
PropertyGroupDescription groupDescription = new PropertyGroupDescription("Group");
EveryoneGrouped.GroupDescriptions.Add(groupDescription);
}
}
XAML
<Window x:Class="FunWithCollectionViewGrouping.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:FunWithCollectionViewGrouping"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525">
<Grid>
<ItemsControl ItemsSource="{Binding EveryoneGrouped}">
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding PersonName}" Margin="5,0"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
<ItemsControl.GroupStyle>
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<TextBlock DockPanel.Dock="Left" FontWeight="Bold" Text="{Binding Name.Group.GroupName}" />
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
</ItemsControl.GroupStyle>
</ItemsControl>
</Grid>
Code Behind
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = new ViewModel();
}
}
Person
public class Person
{
private static int PersonQuantity = 0;
private int _id = ++PersonQuantity;
public int Id { get { return _id; } }
public Group Group { get; set; }
public string PersonName { get; set; }
}
Group
public class Group : ObservableCollection<Person>
{
private static int GroupQuantity = 0;
private int _id = ++GroupQuantity;
public int Id { get { return _id; } }
public string GroupName { get; set; }
public Group()
{
CollectionChanged += Group_CollectionChanged;
}
private void Group_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
if( e.NewItems != null )
{
foreach(Person p in e.NewItems)
{
if ( p.Group != null ) { p.Group.Remove(p); }
p.Group = this;
}
}
if( e.OldItems != null )
{
foreach (Person p in e.OldItems)
{
if (p.Group != null && p.Group.Equals(this)) { p.Group = null; }
}
}
}
public override bool Equals(object obj)
{
Group other = obj as Group;
if (obj == null) return false;
return Id == other.Id;
}
public override int GetHashCode()
{
return Id;
}
}
Thank you.

Answer 1

Group by the GroupName property of the Group:
private void createCollectionView()
{
EveryoneGrouped = (CollectionView)CollectionViewSource.GetDefaultView(Everyone);
PropertyGroupDescription groupDescription = new PropertyGroupDescription("Group.GroupName");
EveryoneGrouped.GroupDescriptions.Add(groupDescription);
}
And display the name of the group:
<ItemsControl.GroupStyle>
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<TextBlock DockPanel.Dock="Left" FontWeight="Bold" Text="{Binding Name}" />
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
</ItemsControl.GroupStyle>
The problem I have with this solution is that multiple groups can have the same name (I updated my post to specify this), and they will be merged in the view if I do this. Groups are distinguished by their ID (which I can't sort by because then I would loose the group name in the view).
Group by the Id property then:
private void createCollectionView()
{
EveryoneGrouped = (CollectionView)CollectionViewSource.GetDefaultView(Everyone);
PropertyGroupDescription groupDescription = new PropertyGroupDescription("Group.Id");
EveryoneGrouped.GroupDescriptions.Add(groupDescription);
}
And bind the TextBlock to the GroupName property of the Group of the first item in the group:
<ItemsControl.GroupStyle>
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<TextBlock DockPanel.Dock="Left" FontWeight="Bold" Text="{Binding Items[0].Group.GroupName}" />
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
</ItemsControl.GroupStyle>
What I don't understand is why it behaves this way, doesn't PropertyGroupDescription use Equals and GetHashCode?
Obviously not. It uses reflection to be able to group by any property specified by a string.