Access a ViewModel from another ViewModel

Question

I'm developing an WPF using MVVM pattern, C# and .NET Framework 4.6.1.

I have a Window that contains an UserControl (Control1) and that UserControl contains another UserControl (Control2). I have chosen this way to do it instead of using a Dialog Window (Control2 acts as Dialog Window).

Both user controls have a Viewmodel (Control1VM and Control2VM).

I use Control2 as a form to let users input some data that I need to start the application.

This is the MainWindow with Control1:

And this is Control2 over Control1.

My problem is that I don't know how to hide Control2 when I click on OK or Cancel button.

This is how Control2 is set on Control1:

<Grid x:Name="gridControl2" Margin="30" Grid.RowSpan="6" Grid.ColumnSpan="3" Visibility="{Binding GridControl2Visibility}">
    <local:Control2 x:Name="userControlControl2" />
</Grid>

To show Control2 and set GridControl2Visibility to Visible in Control1VM:

public Visibility GridControl2Visibility
{
    get { return gridControl2Visibility; }
    set
    {
        if (gridControl2Visibility != value)
        {
            gridControl2Visibility = value;

            RaisePropertyChangedEvent("GridControl2Visibility");
        }
    }
}

How can I hide Control2 when I click on Ok or Cancel button in Control2? My problem is that GridControl2Visibility is on Control1VM and I can't access that class from Control2VM.

The common way to do that would be with the messaging capabilities of your MVVM toolkit. — Lennart
Create a VM for window also and keep instance of Child VM in that. The child VM will in turn consists of its child VM instance. This way, you can control entire code from Window VM. — ViVi

Haukinger Haukinger · Accepted Answer · 2016-09-01T10:26:04

Use a service that both view models can access and that stores the info whether Control2 should be visible or not. Ideally, the service would be registered as singleton with your di-container and injected into the view models.

Alternatively, you can use an event aggregator, which is basically a singleton service, too, but focused on distributing events rather than holding a state.

Access a ViewModel from another ViewModel

2 Answers