Handling Dialogs in WPF with MVVM

Question

In the MVVM pattern for WPF, handling dialogs is one of the more complex operations. As your view model does not know anything about the view, dialog communication can be interesting. I can expose an ICommand that when the view invokes it, a dialog can appear.

Does anyone know of a good way to handle results from dialogs? I am speaking about windows dialogs such as MessageBox.

One of the ways we did this was have an event on the viewmodel that the view would subscribe to when a dialog was required.

public event EventHandler<MyDeleteArgs> RequiresDeleteDialog;

This is OK, but it means that the view requires code which is something I would like to stay away from.

Answer 1

I've written a fairly comprehensive article about this very topic and also developed a pop-in library for MVVM Dialogs. Strict adherence to MVVM is not only possible but very clean when implemented properly, and it can be easily extended to third-party libraries that don't adhere to it themselves:

https://www.codeproject.com/Articles/820324/Implementing-Dialog-Boxes-in-MVVM

Answer 2

Sorry, but I have to chime in. I have been through several of the suggested solutions, before finding the Prism.Wpf.Interactivity namespace in the Prism project. You can use interaction requests and popup window action to either roll a custom window or for simpler needs there are built in Notification and Confirmation popups. These create true windows and are managed as such. you can pass a context object with any dependencies you need in the dialog. We use this solution at my work since I found it. We have numerous senior devs here and noone has come up with anything better. Our previous solution was the dialog service into an overlay and using a presenter class to make it happen, but you had to have factories for all of the dialog viewmodels, etc.

This isn't trivial but it also isn't super complicated. And it is built in to Prism and is therefore best (or better) practice IMHO.

My 2 cents!

Answer 3

A good MVVM dialog should:

1. Be declared with only XAML.
2. Get all of it's behavior from databinding.

Unfortunately, WPF doesn't provide these features. Showing a dialog requires a code-behind call to ShowDialog(). The Window class, which supports dialogs, can't be declared in XAML so it can't easily be databound to the DataContext.

To solve this, I wrote a XAML stub control that sits in the logical tree and relays databinding to a Window and handles showing and hiding the dialog. You can find it here: http://www.codeproject.com/KB/WPF/XAMLDialog.aspx

It's really simply to use and doesn't require any strange changs to your ViewModel and doesn't require events or messages. The basic call looks like this:

<dialog:Dialog Content="{Binding Path=DialogViewModel}" Showing="True" />

You probably want to add a style that sets Showing. I explain it in my article. I hope this helps you.

Answer 4

Use a freezable command

<Grid>
        <Grid.DataContext>
            <WpfApplication1:ViewModel />
        </Grid.DataContext>


        <Button Content="Text">
            <Button.Command>
                <WpfApplication1:MessageBoxCommand YesCommand="{Binding MyViewModelCommand}" />
            </Button.Command>
        </Button>

</Grid>

public class MessageBoxCommand : Freezable, ICommand
{
    public static readonly DependencyProperty YesCommandProperty = DependencyProperty.Register(
        "YesCommand",
        typeof (ICommand),
        typeof (MessageBoxCommand),
        new FrameworkPropertyMetadata(null)
        );


    public static readonly DependencyProperty OKCommandProperty = DependencyProperty.Register(
        "OKCommand",
        typeof (ICommand),
        typeof (MessageBoxCommand),
        new FrameworkPropertyMetadata(null)
        );


    public static readonly DependencyProperty CancelCommandProperty = DependencyProperty.Register(
        "CancelCommand",
        typeof (ICommand),
        typeof (MessageBoxCommand),
        new FrameworkPropertyMetadata(null)
        );


    public static readonly DependencyProperty NoCommandProperty = DependencyProperty.Register(
        "NoCommand",
        typeof (ICommand),
        typeof (MessageBoxCommand),
        new FrameworkPropertyMetadata(null)
        );


    public static readonly DependencyProperty MessageProperty = DependencyProperty.Register(
        "Message",
        typeof (string),
        typeof (MessageBoxCommand),
        new FrameworkPropertyMetadata("")
        );

    public static readonly DependencyProperty MessageBoxButtonsProperty = DependencyProperty.Register(
        "MessageBoxButtons",
        typeof(MessageBoxButton),
        typeof(MessageBoxCommand),
        new FrameworkPropertyMetadata(MessageBoxButton.OKCancel)
        );

    public ICommand YesCommand
    {
        get { return (ICommand) GetValue(YesCommandProperty); }
        set { SetValue(YesCommandProperty, value); }
    }

    public ICommand OKCommand
    {
        get { return (ICommand) GetValue(OKCommandProperty); }
        set { SetValue(OKCommandProperty, value); }
    }

    public ICommand CancelCommand
    {
        get { return (ICommand) GetValue(CancelCommandProperty); }
        set { SetValue(CancelCommandProperty, value); }
    }

    public ICommand NoCommand
    {
        get { return (ICommand) GetValue(NoCommandProperty); }
        set { SetValue(NoCommandProperty, value); }
    }

    public MessageBoxButton MessageBoxButtons
    {
        get { return (MessageBoxButton)GetValue(MessageBoxButtonsProperty); }
        set { SetValue(MessageBoxButtonsProperty, value); }
    }

    public string Message
    {
        get { return (string) GetValue(MessageProperty); }
        set { SetValue(MessageProperty, value); }
    }

    public void Execute(object parameter)
    {
        var messageBoxResult = MessageBox.Show(Message);
        switch (messageBoxResult)
        {
            case MessageBoxResult.OK:
                OKCommand.Execute(null);
                break;
            case MessageBoxResult.Yes:
                YesCommand.Execute(null);
                break;
            case MessageBoxResult.No:
                NoCommand.Execute(null);
                break;
            case MessageBoxResult.Cancel:
                if (CancelCommand != null) CancelCommand.Execute(null); //Cancel usually means do nothing ,so can be null
                break;

        }
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public event EventHandler CanExecuteChanged;


    protected override Freezable CreateInstanceCore()
    {
        throw new NotImplementedException();
    }
}

Answer 5

I've implemented a Behavior that listens to a Message from the ViewModel. It's based on Laurent Bugnion solution, but since it doesn't use code behind and is more reusable, I think it's more elegant.

How to make WPF behave as if MVVM is supported out of the box