WPF label not updating value on the UI

Question

Hi All,
I am currently working on a C# WPF application. The UI contains a tree view with checkboxes. I need a label on the UI to update the number of checked checkboxes. However, even though i get the value and try to update the UI, the 'Property Changed' from the INotifyPropertyChanged is coming as null and preventing the UI from being updated.

I have attached the code. The checkbox in question is the IsChildChecked checkbox and the Label that needs to be updated is the SelectedDevicesCount. The constants.counter is a public static variable which holds on to the value of checked checkboxes.7815-label-and-checkbox-issue.txt
Can anyone please help me on this??

Thank you in advance!!

Answer 1

Hi; you calculate SelectedCount in ViewModel like in following demo:

<Window x:Class="WpfApp1.Window35"
        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:WpfApp35"
        mc:Ignorable="d"
        Title="Window35" Height="450" Width="800">
  <Window.Resources>
    <local:ViewModel x:Key="vm"/>
    <Style x:Key="HighlightingTextStyle" TargetType="Label">
      <Setter Property="FontStyle" Value="Italic"/>
    </Style>
  </Window.Resources>

  <Grid DataContext="{StaticResource vm}">
    <StatusBar VerticalAlignment="Top">
      <Label Content="Selected Devices:" Margin="1,1,0,1"/>
      <Label Content="{Binding SelectedDevicesCount, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    </StatusBar>
    <TreeView ItemsSource="{Binding TreeViewDataCollection}" Margin="0 40 0 0">
      <TreeView.Resources>
        <HierarchicalDataTemplate ItemsSource="{Binding HostGroupName}" DataType="{x:Type local:RootNameList}">
          <StackPanel Orientation="Horizontal">
            <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding RootSubnet}"/>
          </StackPanel>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Host}" DataType="{x:Type local:HostGroup}">
          <StackPanel Orientation="Horizontal">
            <CheckBox IsChecked="{Binding IsParentChecked}" IsThreeState="True"/>
            <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding GroupName}" Tag="{Binding RelativeSource={RelativeSource AncestorType=Window}}"/>
          </StackPanel>
        </HierarchicalDataTemplate>
        <HierarchicalDataTemplate DataType="{x:Type local:HostName}">
          <StackPanel Orientation="Horizontal">
            <CheckBox IsChecked="{Binding IsChildChecked}"/>
            <Label Style="{StaticResource HighlightingTextStyle}" Content="{Binding Host}" Tag="{Binding RelativeSource={RelativeSource AncestorType=Window}}"/>
          </StackPanel>
        </HierarchicalDataTemplate>
      </TreeView.Resources>
    </TreeView>
  </Grid>
</Window>

Classes:

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Data;


namespace WpfApp35
{
  public class ViewModel : BaseClass
  {
    public ViewModel()
    {
      LoadData();
      SetHandler();
      cvs.Source = col;
    }
    private void SetHandler()
    {
      foreach (var rnl in col)
        foreach (var hg in rnl.HostGroupName)
        {
          hg.SetHandler();
          hg.PropertyChanged += Hg_PropertyChanged;
        }
    }

    private void Hg_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
      SelectedDevicesCount = 0;
      foreach (var rnl in col)
        foreach (var hg in rnl.HostGroupName) SelectedDevicesCount += hg.SelectedCount;
      OnPropertyChanged(nameof(SelectedDevicesCount));
    }

    private void LoadData()
    {
      for (int i = 1; i < 10; i++)
      {
        var rnl = new RootNameList { RootSubnet = $"RootSubnet{i}" };
        for (int k = 0; k < 10; k++)
        {
          var hg = new HostGroup { GroupName = $"GroupName {k + i * 10}" };
          rnl.HostGroupName.Add(hg);
          for (int l = 0; l < 10; l++)
          {
            var h = new HostName() { Host = $"Host {l}" };
            hg.Host.Add(h);
          }
        }
        col.Add(rnl);
      }
    }

    public int SelectedDevicesCount { get; set; }

    private CollectionViewSource cvs = new CollectionViewSource();
    private ObservableCollection<RootNameList> col = new ObservableCollection<RootNameList>();

    public ICollectionView TreeViewDataCollection { get => cvs.View; }
  }

  public class RootNameList
  {
    public string RootSubnet { get; set; }
    public List<HostGroup> HostGroupName { get; set; } = new List<HostGroup>();
  }

  public class HostGroup : BaseClass
  {

    private bool? _IsParentChecked = false;
    public bool? IsParentChecked
    {
      get => this._IsParentChecked;
      set
      {
        if (!value.HasValue) value = false;
        this._IsParentChecked = value;
        foreach (var h in Host) h.IsChildChecked = value.Value;
        OnPropertyChanged();
      }
    }
    public string GroupName { get; set; }
    public List<HostName> Host { get; set; } = new List<HostName>();

    public void SetHandler()
    {
      foreach (var hn in Host) hn.PropertyChanged += h_PropertyChanged;
    }

    public int SelectedCount { get; set; } = 0;

    internal void h_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
      int iFalse = 0;
      SelectedCount = 0;
      foreach (var h in Host) if (h.IsChildChecked) SelectedCount++; else iFalse++;
      if (SelectedCount > 0 && iFalse == 0) this._IsParentChecked = true;
      if (SelectedCount == 0 && iFalse > 0) this._IsParentChecked = false;
      if (SelectedCount > 0 && iFalse > 0) this._IsParentChecked = null;
      OnPropertyChanged(nameof(IsParentChecked));
    }
  }

  public class HostName : BaseClass
  {

    private bool _isChildChecked = false;
    public bool IsChildChecked
    {
      get => this._isChildChecked;
      set
      {
        this._isChildChecked = value;
        OnPropertyChanged();
      }
    }
    public string Host { get; set; }
    public string HostType { get; set; }
  }

  public class BaseClass : INotifyPropertyChanged
  {
    public event PropertyChangedEventHandler PropertyChanged;
    internal void OnPropertyChanged([CallerMemberName] string nameProp = "") =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameProp));
  }
}