Setting datacontext inside ViewModel

Question

I have a MainWindowView that has a grid with 2 columns each having 1 UserControl View. MainWindowView constructor creates instance of MainWindowVM and sets the data context to this new instance.

this.DataContext = new MainWindowVM(this)

Now question is I am trying to set data context of each UserControlView to an instance of it's respective ViewModel inside MainWindowVM. How can I access the UserControlView inside MainWindowVM to do something like this

UserControl1View.DataContext= new UserControl1ViewModel()

If I can do this, it will allow me to use MainWindowVM as a common hub holding all kinds of event subscriptions from the 2 userControls.

Don't do this. MainWindowVM should expose properties for its child view model. Bind a UserControl's DataContext (or that of its parent element) to such a child view model in XAML, like `view1.DataContext="{Binding View1VM}"` — Clemens, Mar 31 '18 at 06:03
Where View1VM is a bindable property of MainWindowViewModel. — Andy, Mar 31 '18 at 10:08

score 0 · Answer 1 · answered Mar 31 '18 at 05:16

0

ViewModel must not depend upon View, and both must have one-to-one relationship between them. So best is to use Binding to set DataContext and if this setting of DataContext depends upon some condition, then use Triggers.

answered Mar 31 '18 at 05:16

AnjumSKhan

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score 0 · Answer 2 · answered Apr 02 '18 at 05:24

Don't-Do-That.

A better approach is to have a ViewModel reference in he View.

Create an interface similar to this:

public interface IView<T> where T : class
{
    T ViewModel;
}

Now, your Views must implement that interface

public partial class MainView : Window, IView<MainViewModel>
{
    public MainViewModel ViewModel { get; set; }

And inject the ViewModel in the view constructor:

public MainView(MainViewModel vm)
{
    this.ViewModel = vm;
    this.DataContext = this.ViewModel;
    // you can create the VMs you want for the another views
    var vm1 = new UserControl1ViewModel();
    // and pass it to the UserControl1View (UserControl1View implements IView<T>
    var view1 = new UserControl1View(vm1);

Setting datacontext inside ViewModel

2 Answers2