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

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>