Getting Window from view model

Question

I have created a custom messageBox Window to replace the typical MessageBox.

My custom messageBox (child Window) needs parent window to be passed as an argument. The parent window is where child window will be located within in the location specified (Top Left, Top Center, etc.) also as an argument.

So when calling my custom messageBox from view model, I need to get the Window to pass it through. How can I get the Window associated to the view model?

Maybe using an Interface like commented here? I am trying to implement it but this.DataContext.View does not exist.

I am using Visual Studio 2008.

ATTEMPT #1: mm8 solution

<Window x:Class="MyNamespace.Main"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:viewmodel="clr-namespace:MyNamespace.ViewModels">
    <Window.Resources>
        <viewmodel:MainViewModel x:Key="wMainViewModel" />
    </Window.Resources>

    <Grid DataContext="{StaticResource wMainViewModel}">
    </Grid>
</Window>

Taken into account that I initialize DataContext from xaml and not in code-behind from constructor, how can I pass the view to the view model¿?

isn't code that handles messageboxes etc. the *other* part of MVVM? (as in ; it has no business in your model) — Timothy Groote, Nov 13 '17 at 14:07
Either you pass it to the ViewModel or use `Application.Current.MainWindow`. But yeah, as Timothy said, your ViewModel has no business knowing about Windows. — Manfred Radlwimmer, Nov 13 '17 at 14:08
Possible duplicate of [how to show Messagebox in MVVM](https://stackoverflow.com/questions/3705991/how-to-show-messagebox-in-mvvm) — Sinatr, Nov 13 '17 at 14:08
@ManfredRadlwimmer using Application.Current.MainWindow is ok when you have only one window but when you have a lot it is not suitable. — Willy, Nov 13 '17 at 14:20
@user1624552 Then use `Application.Current.Windows.OfType().FirstOrDefault()` or use MVVM properly :) — Manfred Radlwimmer, Nov 13 '17 at 14:23

score 8 · Accepted Answer · edited Nov 14 '17 at 12:04

8

How can I get the Window associated to the view model?

You could inject the view model with an interface that the window implements:

public interface IView
{
    double Top { get; }
    double Left { get; }
    double Height { get; }
    double Width { get; }
}

public partial class MainWindow : Window, IView
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new ViewModel(this);
    }
}

public class ViewModel
{
    private readonly IView _view;

    public ViewModel(IView view)
    {
        _view = view;
    }

    //...
}

Then the view model knows only about an interface (which maybe should be called something else than IView).

If you want an ugly and non-MVVM friendly shortcut, you could use the Application class:

var win = Application.Current.MainWindow;

A factory class to create everything:

public MainWindow WindowFactory()
{
    MainWindow mainWindow = new MainWindow();
    ViewModel viewModel = new ViewModel();
    mainWindow.DataContext = viewModel;

    return mainWindow;
}

edited Nov 14 '17 at 12:04

Ignacio Soler Garcia

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answered Nov 13 '17 at 14:31

mm8

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See my update (Attempt #1). I initialize datacontext from xaml, not code-behind so how can I pass the view in this case? – Willy Nov 13 '17 at 15:06
2

You can't. Creating the view model in XAML is a bad idea, especially in this case. Create it in the code-behind instead. – mm8 Nov 13 '17 at 15:07
Ok, and can I have datacontext initialized in xaml as I have posted and also in code-behind as you said? – Willy Nov 13 '17 at 15:10
No. Then you would be creating two instances of the view model class. – mm8 Nov 13 '17 at 15:10
Perfect! A lot of thanks, your solution works perfectly! Many thanks for your great explanation and example. – Willy Nov 13 '17 at 15:18
Even better create the view and the viewmodel in a factory and set there the datacontext. – Ignacio Soler Garcia Nov 13 '17 at 17:27
@IgnacioSolerGarcia Out of curiosity, could you provide a little example? – Willy Nov 13 '17 at 17:43
@user1624552: its trivial. Added to the end of the answer. – Ignacio Soler Garcia Nov 14 '17 at 12:05

Getting Window from view model

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