When the SelectedItems attached property stops to work when I change the tab?

Question

I have a view and a view model. I have a datagrid that when I select a row, a textbox shows the value of the row. This is just to show if the selectedItems it is fired.

it works fine, but if I change to the other tab and back, it stops to work.

I am using Net 5.

My view:

<Window x:Class="TabControlError.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:TabControlError"
        xmlns:vm="clr-namespace:TabControlError.ViewModels"
        xmlns:dp="clr-namespace:TabControlError.DependencyProperties"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <Window.DataContext>
        <vm:MainWindowViewModel/>
    </Window.DataContext>



    <Grid>
        <TabControl>
            <TabItem Header="Tab1">
                <Grid>
                    <TextBox Text="{Binding Text}" Width="100" Height="23" HorizontalAlignment="Left" VerticalAlignment="Top"/>

                    <DataGrid Margin="0,40,0,0"
                              ItemsSource="{Binding Numbers}"
                              SelectedItem="{Binding NumbersSelectedItem}"
                              dp:DataGridSelectedItemsDependencyProperty.SelectedItems="{Binding NumbersSelectedItems}">

                        <DataGrid.Columns>
                            <DataGridTextColumn Header="Number" Binding="{Binding Path=.}" Width="150"/>
                        </DataGrid.Columns>
                    </DataGrid>
                </Grid>
            </TabItem>

            <TabItem Header="Tab2"/>
        </TabControl>
    </Grid>
</Window>

My view model base:

using System.ComponentModel;
using System.Runtime.CompilerServices;



namespace TabControlError
{
    class ViewModelBase : INotifyPropertyChanging, INotifyPropertyChanged
    {
        #region INotifyPropertyChanging Members

        public event PropertyChangingEventHandler PropertyChanging;

        #endregion

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion

        #region Administrative Properties

        /// <summary>
        /// Whether the view model should ignore property-change events.
        /// </summary>
        public virtual bool IgnorePropertyChangeEvents { get; set; }

        #endregion

        #region Public Methods

        /// <summary>
        /// Raises the PropertyChanged event.
        /// </summary>
        /// <param name="propertyName">The name of the changed property.</param>
        //NOTA: el atributo CallerMemberName no es necesario, pero permite escribir OnPropertyChanged() en lugar de
        //OnPropertyChanged("SomeProperty"), por lo que se evita utilizar strings en el código.
        public virtual void RaisePropertyChangedEvent([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        /// <summary>
        /// Raises the PropertyChanging event.
        /// </summary>
        /// <param name="propertyName">The name of the changing property.</param>
        //NOTA: el atributo CallerMemberName no es necesario, pero permite escribir OnPropertyChanged() en lugar de
        //OnPropertyChanged("SomeProperty"), por lo que se evita utilizar strings en el código.
        public virtual void RaisePropertyChangingEvent(string propertyName)
        {
            PropertyChanging?.Invoke(this, new PropertyChangingEventArgs(propertyName));
        }

        #endregion
    }
}

My view model:

using System;
using System.Linq;
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.Text;

namespace TabControlError.ViewModels
{
    class MainWindowViewModel : ViewModelBase
    {
        public MainWindowViewModel()
        {
            Numbers.Add(0);
            Numbers.Add(1);
            Numbers.Add(2);
            Numbers.Add(3);
        }




        private string _text;
        public string Text
        {
            get { return _text; }
            set
            {
                if (_text != value)
                {
                    _text = value;
                    RaisePropertyChangedEvent();
                }
            }
        }



        private ObservableCollection<long> _numbers = new ObservableCollection<long>();
        public ObservableCollection<long> Numbers
        {
            get { return _numbers; }
            set
            {
                if (_numbers != value)
                {
                    _numbers = value;
                    RaisePropertyChangedEvent();
                }
            }
        }

        private long? _numbersSelectedItem;
        public long? NumbersSelectedItem
        {
            get { return _numbersSelectedItem; }
            set
            {
                if (_numbersSelectedItem != value)
                {
                    _numbersSelectedItem = value;
                    RaisePropertyChangedEvent();
                }
            }
        }

        private ObservableCollection<object> _numbersSelectedItems = new ObservableCollection<object>();
        public ObservableCollection<object> NumbersSelectedItems
        {
            get { return _numbersSelectedItems; }
            set
            {
                _numbersSelectedItems = value;
                Text = NumbersSelectedItem?.ToString();
                RaisePropertyChangedEvent();
            }
        }
    }
}

My attached property:

using System.Windows;
using System.Collections;
using System.Windows.Controls;

namespace TabControlError.DependencyProperties
{
    class DataGridSelectedItemsDependencyProperty
    {
        #region SelectedItems
        ///
        /// SelectedItems Attached Dependency Property
        ///
        public static readonly DependencyProperty SelectedItemsProperty =
        DependencyProperty.RegisterAttached("SelectedItems", typeof(IList),
        typeof(DataGridSelectedItemsDependencyProperty),
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
        new PropertyChangedCallback(OnSelectedItemsChanged)));

        public static IList GetSelectedItems(DependencyObject d)
        {
            return (IList)d.GetValue(SelectedItemsProperty);
        }

        public static void SetSelectedItems(DependencyObject d, IList value)
        {
            d.SetValue(SelectedItemsProperty, value);
        }

        private static void OnSelectedItemsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {   
            DataGrid miDataGrid = (DataGrid)d;
            miDataGrid.SelectionChanged += DataGrid_SelectionChanged;
            miDataGrid.Unloaded += dataGrid_Unloaded;
        }

        private static void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            DataGrid miDatagrid = (DataGrid)sender;

            IList ModelSelectedItems = GetSelectedItems(miDatagrid);

            ModelSelectedItems.Clear();

            if (miDatagrid.SelectedItems != null)
            {
                foreach (var item in miDatagrid.SelectedItems)
                    ModelSelectedItems.Add(item);
            }
            SetSelectedItems(miDatagrid, ModelSelectedItems);
        }


        private static void dataGrid_Unloaded(object sender, RoutedEventArgs e)
        {   
            DataGrid miDg = sender as DataGrid;
            miDg.SelectionChanged -= DataGrid_SelectionChanged;
            miDg.Unloaded -= dataGrid_Unloaded;
        }
        #endregion
    }
}

Why does it stop working when I change the tab in the tab control?

Thanks.

Answer 1

I tested your code and can reproduce your error. To anlayze the error, I cut Text = NumbersSelectedItem?.ToString(); from line 73 and insert it between line 60 and line 61 for your third part code (MainWindowViewModel ) in the question, then the project worked well. From this, we can think problem comes from your custom DependencyProperty SelectedItems.

I check your DataGridSelectedItemsDependencyProperty and found that the dataGrid_Loaded method was missing from it. I add dataGrid_Loaded in your OnSelectedItemsChanged and implement dataGrid_Loaded in the class like below shown:

private static void OnSelectedItemsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)  
        {  
            DataGrid miDataGrid = (DataGrid)d;  
            miDataGrid.SelectionChanged += DataGrid_SelectionChanged;  
            miDataGrid.Unloaded += dataGrid_Unloaded;  
            miDataGrid.Loaded += dataGrid_Loaded; ;  
        }  
  
        private static void dataGrid_Loaded(object sender, RoutedEventArgs e)  
        {  
            DataGrid miDg = sender as DataGrid;  
            miDg.SelectionChanged += DataGrid_SelectionChanged;  
            miDg.Loaded += dataGrid_Loaded;  
        }  

After doing this, the set method of NumbersSelectedItems will be called and the program can run well when the window reloads.

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