WPF -Child datacontext set from the parent ViewModel doesnot contains the modified values in the parent ViewModel

Question

In my WPF application I have a MainWindow.xaml which has many User Controls embedded into it. I have separate viewmodel for both the main window as well as the user controls. In the Main Window ViewModel I create an instance of the child ViewModel and set it as the child's datacontext.

Parent VM ChildUserControlVM ChildVM = new ChildUserControlVM (ParentModel.ChildModel);

MainWindow.xaml

But this approach is not working as I am not getting the values set in the parent viewmodel in the child one and vice versa.

On the contrary, if I set the Child model object as the datacontext from the parent that is working both way.

I need some solution so that I can use MVVM in the user controls also ensuring data gets passed from parent to child and vice versa. In the usercontrol I am going to have some buttons whose action I want to handle in the child Viewmodel through ICommand. Adding the code snippets for reference MainWindow.xaml

<Grid Grid.Row="0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,0,0,0">
    <local:ProfileIdentitySettings Visibility="{Binding ProfileIdentitySettingsVisibility,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}" DataContext="{Binding ChildProfileIdentityVM}"/>      
</Grid>

MainWindowVM.cs

 ProfileIdentitySettingsVM ChildProfileIdentityVM = new ProfileIdentitySettingsVM(DeviceEditorModel.ProfileIdentitySettings);

ProfileIdentitySettings.xaml

DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.MainWindowVM}">

Answer 1

The parent view model should contain the child view models as properties, like

public class ParentViewModel
{
    public ChildViewModel1 ChildViewModel1 { get; set; }
    public ChildViewModel2 ChildViewModel2 { get; set; }
}

Your UserControls should not have any view models of their own. Instead they should expose dependency properties like

public partial class MyView1 : UserControl
{
    public string MyProperty { get; set; } // this must be a dependency property
}

which can be bound when the UserControl is used, e.g. in your MainWindow's XAML, like

<MyView1 MyProperty="{Binding ChildViewModel1.SomeProperty}" />

If there are many of these bindable properties, you may set the UserControl's DataContext once to the appropriate child view model:

<MyView1 DataContext="{Binding ChildViewModel1}"
         MyProperty="{Binding SomeProperty}" />

Answer 2

Set the datacontext of each of your usercontrol in the xaml

<UserControl.DataContext>
    <VMPath:ChildVM/>
</UserControl.DataContext>

then add a loaded event for your user controls defined in your MainWindow

<ViewPath:MyUserControl x:Name="uc1" Loaded="UserControlLoaded"/>

in your code behind you'll have

    private void UserControlLoaded(object sender, RoutedEventArgs e)
    {
        ParentVM vm = this.DataContext as VM;
        ChildVM cvm = uc1.DataContext as ChildVM;
        // do your stuff
    }

if the operation you need to do are always the same you can use 1 method to all of your UserControl.

Basically what you are going to do are the following operation.

• set statically the dantacontext for your user controls to an instance of an "empty" ChildVM

• load your controls

• after the loading do the operation you need to do on your ChildVM (like set some properties)

All of this before you can actually use your app

Answer 3

My Understanding is like you need to set the usercontrol Viewmodel from MainViewModel.

MainViewModel:

 public class MainWindowViewModel : INotifyPropertyChanged, IMainViewModel
{

    public MainWindowViewModel()
    {
        this.Collection = new List<string>();
        this.Child = new ChildUserControlViewModel();
        this.Child.mainViewModel = this;
        this.Child.TextValue = "Justin";
    }
    private List<string> _Collection;

    public List<string> Collection
    {
        get { return _Collection; }
        set { _Collection = value; this.OnPropertyChanged("Collection"); }
    }
    private string _MainValue;
    public string MainValue
    {
        get { return _MainValue; }
        set { _MainValue = value; this.OnPropertyChanged("MainValue"); }
    }
    private ChildUserControlViewModel child;
    public ChildUserControlViewModel Child
    {
        get { return child; }
        set { child = value; this.OnPropertyChanged("Child"); }
    }

}

Child Usercontrol View model:

public class ChildUserControlViewModel : INotifyPropertyChanged
{

    public IMainViewModel mainViewModel = null;

    public List<string> Collection
    {
        get { return this.mainViewModel.Collection; }
        set { this.mainViewModel.Collection = value; this.OnPropertyChanged("Collection"); }
    }

    private string _TextValue;
    public string TextValue
    {
        get { return _TextValue; }
        set
        {
            _TextValue = value;
            this.mainViewModel.MainValue = value;
            this.mainViewModel.Collection.Add(value);
            this.OnPropertyChanged("TextValue");
        }
    }
}





 public interface IMainViewModel
    {
        string MainValue { get; set; }

         List<string> Collection { get; set; }
    }

View:

<Grid>
   <usr:ChildUserControl DataContext="{Binding Child}"/>
</Grid>