Databindings don't seem to refresh in MVVM

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I am trying to rewrite my CheckBoxList from Code Behind to MVVM pattern in WPF C#. The thing is that now I have a problem to get all selected checkboxes. I have implemented INotifyPropertyChanged interface. Project is building correctly, when I set the break point I can notice that I received always false value for my checkboxes, even if they are selected. I assume that maybe I did something wrong with data binding. Please, does anybody can help? I am totally newbie in MVVM.

What I have tried:

 public class ObservableObject : INotifyPropertyChanged
    {

        #region INotifyPropertyChanged

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string strPropertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(strPropertyName));
        }

        #endregion

    }
}

public class SharedModel : ObservableObject
   {
               private bool _fIsSelected;

               public bool IsSelected
               {
                   get => _fIsSelected;
                   set
                   {
                       _fIsSelected = value;
                       OnPropertyChanged("IsSelected");
                   }
               }

       public bool IsSelected { get; set; }

       public string Name { get; set; }

       public string Method { get; set; }
   }

class TestViewModel : ObservableObject
    {

        public ObservableCollection<SharedModel> List { get; set; } = new ObservableCollection<SharedModel>
        {
            new SharedModel
            {
                Name = "A1",
                Method = Test(),
            },
            new SharedModel
            {
                Name = "A2",
                Method = TestOne()
            }
};

        public string GetSelectedCheckboxes()
        {
             var command =
                    from item in List
                    where item.IsSelected
                    select item.Method;
          return string.Join("\r&", new NewList<string>(command));
        }

<StackPanel Margin="0,0,769,510">
            <ListBox Name="ListBox"
                     ScrollViewer.HorizontalScrollBarVisibility="Disabled"
                     ItemsSource="{Binding List}"
                     SelectionMode="Multiple" Background="{x:Null}" Margin="0,133,590,470" Foreground="White" BorderBrush="{x:Null}">
                <ListBox.ItemsPanel>
                    <ItemsPanelTemplate>
                        <WrapPanel />
                    </ItemsPanelTemplate>
                </ListBox.ItemsPanel>
                <ListBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal"
                                    MinWidth="170" MaxWidth="170"
                                    Margin="0,0, 0, 0" >
                            <CheckBox x:Name="TestCheckbox"
                                      Tag="{Binding Method}" IsChecked="{Binding IsSelected}" />
                            <ContentPresenter
                                Content="{Binding Name}"
                                Margin="5,0, 15, 0" />
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </StackPanel>

Posted 8-Feb-18 8:08am

Member 13668663

Updated 8-Feb-18 8:57am

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George Swan 10-Feb-18 2:39am

The IsSelected property needs to call OnPropertyChanged() in its setter

1 solution

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J. Calhoun · Accepted Answer · 2018-02-08T08:57:00

First, you don't implement OnChanged event in your model, only the view model.
Next, one thing you have to be careful with in databinding is the "new" keyword.

The practice that I have found to work the best for me is declare the variable

privateObservableCollection<SharedModel> _List;
public ObservableCollection<SharedModel> List 
{ 
    get { return _List; } 
    set { _List = value; OnPropertyChanged("List");} 
}

Then I initialize it in the constructor so that it is only using the "new" keyword one time. The reason for this is that the "new" keyword will often "break" the binding. I am not sure if you want to modify the list from the viewmodel, so I usually specify Two-Way for the mode so that I can change items from both the view and viewmodel. I also change the update source trigger, but IIRC lostfocus is the default.

XML

<Listview Itemssource="{Binding List, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">

Then in your item template, your IsSelected should update.

<ListBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal"

                                    MinWidth="170" MaxWidth="170"

                                    Margin="0,0, 0, 0" >
                            <CheckBox x:Name="TestCheckbox"

                                      Tag="{Binding Method}" IsChecked="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
                            <ContentPresenter

                                Content="{Binding Name}"

                                Margin="5,0, 15, 0" />
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>

I don't know if you use a command, or event or whatever you do to get to GetSelectedCheckboxes(), but there are a million different ways to return all of the true selections. one way might be:

C#

public string GetSelectedCheckboxes()
        {
             List<string> yourlist = List.where(l => l.IsSelected == true).Select(x => x.Method).ToList();
             return yourlist;   
        }

Hope this helps, if you need me to clarify further I will just let me know.

Edit: To show you how I set it up.

public class SharedModel
   {
       public bool IsSelected { get; set; }

       public string Name { get; set; }

       public string Method { get; set; }
   }

C#

public class TestViewModel : ObservableObject
{
    privateObservableCollection<SharedModel> _List;
    public ObservableCollection<SharedModel> List 
    { 
        get { return _List; } 
        set { _List = value; OnPropertyChanged("List");} 
    }

    public TestViewModel()
    {
        List = new ObservableCollection<SharedModel>()
        {
            new SharedModel()
            {
                Name = "A1",
                IsSelected = false
            },
            new SharedModel()
            {
                Name = "A2",
                IsSelected = false
            }
        };
    }

    public List<string> GetSelectedCheckboxes()
    {
        List<string> selectedCheckboxes = List.Where(x => x.IsSelected == true).Select(x => x.Name).ToList();
        return selectedCheckboxes;
}

This along with you setting the DataContext should work. All you have to do is wire up a button to call GetSelectedCheckboxes() and that list will be only checkboxes that are selected.