Multiple instances of a WPF user control all use the same ViewModel

Question

I have an app that when I click a button loads a new TabItem with a UserControl. This works fine, but... When I click the Add button again, another TabItem is added with a new instance of the UserControl. Here comes the problem. When I add some data in tab1 and switch to tab2, I also see the entered data there. How is this possible? I use a new ViewModel for each UserControl so how can both user controls on tab1 and tab2 show the same data. It will probably be something stupid, but I can't find it...

Structure is like this: MainWindow loads a UserControl with only a TabControl on it. After clicking an Add button in the menu a new TabItem is added with a new UserControl on it.

There are here on SO some issues that look like this but are different.

Here's some code.

TabControl.xaml:

<Grid>
    <TabControl x:Name="tabControl" ItemsSource="{Binding TabItems}" SelectedIndex="0">
        <TabControl.Resources>
            <DataTemplate DataType="{x:Type ViewModel:OverviewViewModel}">
                <my:OverviewControl />
            </DataTemplate>
            <DataTemplate DataType="{x:Type ViewModel:MemberViewModel}">
                <my:MemberControl />
            </DataTemplate>
        </TabControl.Resources>
        <TabControl.ItemContainerStyle>
            <Style TargetType="TabItem">
                <Setter Property="Header" Value="{Binding Header}" />
            </Style>
        </TabControl.ItemContainerStyle>
    </TabControl>
</Grid>

TabControl.cs:

public TabControl()
{
    InitializeComponent();
    this.DataContext = new TabViewModel(true);
}

TabViewModel has a DP which holds the TabItems:

public static readonly DependencyProperty TabItemsProperty =
    DependencyProperty.Register(
    "TabItems", typeof(ObservableCollection<ITabViewModel>), typeof(TabViewModel),
    new UIPropertyMetadata(new ObservableCollection<ITabViewModel>()));
public ObservableCollection<ITabViewModel> TabItems
{
    get { return (ObservableCollection<ITabViewModel>)GetValue(TabItemsProperty); }
    set { SetValue(TabItemsProperty, value); }
}

The UserControl which gets loaded on the TabItem, MemberControl.cs:

public MemberControl()
{
  InitializeComponent();
    this.DataContext = new MemberViewModel{};
}

The MemberControlViewModel:

private MemberModel _memberModel = new MemberModel();

public MemberViewModel()
{
  Address = new AddressViewModel();
}

public static readonly DependencyProperty FirstNameProperty =
    DependencyProperty.Register("FirstName", typeof(string), typeof(MemberViewModel),
    new UIPropertyMetadata(string.Empty, OnFirstNameChanged));
public string FirstName
{
    get { return (string)GetValue(FirstNameProperty); }
    set { SetValue(FirstNameProperty, value); }
}

public static void OnFirstNameChanged(DependencyObject m,
    DependencyPropertyChangedEventArgs e)
{
    var d = m as MemberViewModel;
    if (d._memberModel != null)
        d._memberModel.FirstName = d.FirstName;
}

public static readonly DependencyProperty LastNameProperty =
    DependencyProperty.Register("LastName", typeof(string), typeof(MemberViewModel),
    new UIPropertyMetadata(string.Empty, OnLastNameChanged));
public string LastName
{
    get { return (string)GetValue(LastNameProperty); }
    set { SetValue(LastNameProperty, value); }
}

public static void OnLastNameChanged(DependencyObject m,
    DependencyPropertyChangedEventArgs e)
{
    var d = m as MemberViewModel;
    if (d._memberModel != null)
        d._memberModel.LastName = d.LastName;
}

And then at last the Command on my menu that actually adds the MemberControl as an new TabItem on the TabControl:

private void LoadNewMember(object notUsed)
{
    _tabViewModel.TabItems.Add(new MemberViewModel { Header = "New Member" });
    _addOverview.RaiseCanExecuteChanged();
}

Somewhere along the way I do something wrong with the binding, but as said, I can't find it.

Thanks for any help.

Answer 1

You appear to be setting the DataContext of each MemberControl twice.

Once when you add the MemberViewModel to your tab collection:

_tabViewModel.TabItems.Add(new MemberViewModel { Header = "New Member" });

this will create a MemberControl due to the DataTemplate you have set up.

Then, in your MemberControl's constructor you then reset the DataContext to a new instance of MemberViewModel:

public MemberControl()
{
    InitializeComponent();
    this.DataContext = new MemberViewModel{};
}

I assume you are getting default values in both tabs due to this second new.