WPF MVVM Modal Overlay Dialog only over a View (not Window)

Question

I'm pretty much new to the MVVM architecture design...

I was struggling lately to find a suitable control already written for such a purpose but had no luck, so I reused parts of XAML from another similar control and got make my own.

What I want to achieve is:

Have a reusable View (usercontrol) + viewmodel (to bind to) to be able to use inside other views as a modal overlay showing a dialog that disables the rest of the view, and shows a dialog over the it.

How I wanted to implement it:

• create a viewmodel that takes string(message) and action+string collection(buttons)
• viewmodel creates a collection of ICommands that call those actions
• dialog view binds to the its viewmodel that will be exposed as property of another viewmodel (parent)
• dialog view is put into the xaml of the parent like this:

pseudoXAML:

    <usercontrol /customerview/ ...>
       <grid>
         <grid x:Name="content">
           <various form content />
         </grid>
         <ctrl:Dialog DataContext="{Binding DialogModel}" Message="{Binding Message}" Commands="{Binding Commands}" IsShown="{Binding IsShown}" BlockedUI="{Binding ElementName=content}" />
      </grid>
    </usercontrol>

So here the modal dialog gets the datacontext from the DialogModel property of the Customer viewmodel, and binds commands and message. It would be also bound to some other element (here 'content') that needs to be disabled when the dialog shows (binding to IsShown). When you click some button in the dialog the associated command is called that simply calls the associated action that was passed in the constructor of the viewmodel.

This way I would be able to call Show() and Hide() of the dialog on the dialog viewmodel from inside the Customer viewmodel and alter the dialog viewmodel as needed.

It would give me only one dialog at a time but that is fine. I also think that the dialog viewmodel would remain unittestable, since the unittests would cover the calling of the commands that ought to be created after it being created with Actions in the constructor. There would be a few lines of codebehind for the dialog view, but very little and pretty dumb (setters getters, with almost no code).

What concerns me is:

Is this ok? Are there any problems I could get into? Does this break some MVVM principles?

Thanks a lot!

EDIT: I posted my complete solution so you can have a better look. Any architectural comments welcome. If you see some syntax that can be corrected the post is flagged as community wiki.

Passed

What about a styled/templated BusyIndicator? Does this exist in WPF? I think a ChildWindow is not what you want – Garik

What I want is some kind of MessageBox that does not block the whole Window but just the View in which it is applied. Having a busy indicator is merely a matter of style (ie. The Messagebox is not much different from a busy indicator in that case - since the parent viewmodel can use Hide and SHow - just a variation of the MessageBox without message and commands - call it WaitBox). – Bullington

Answer 1

Well not exactly an answer to my question, but here is the result of doing this dialog, complete with code so you can use it if you wish - free as in free speech and beer:

XAML Usage in another view (here CustomerView):

<UserControl 
  x:Class="DemoApp.View.CustomerView"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:controls="clr-namespace:DemoApp.View"
  >
  <Grid>
    <Grid Margin="4" x:Name="ModalDialogParent">
      <put all view content here/>
    </Grid>
    <controls:ModalDialog DataContext="{Binding Dialog}" OverlayOn="{Binding ElementName=ModalDialogParent, Mode=OneWay}" IsShown="{Binding Path=DialogShown}"/>    
  </Grid>        
</UserControl>

Triggering from parent ViewModel (here CustomerViewModel):

  public ModalDialogViewModel Dialog // dialog view binds to this
  {
      get
      {
          return _dialog;
      }
      set
      {
          _dialog = value;
          base.OnPropertyChanged("Dialog");
      }
  }

  public void AskSave()
    {

        Action OkCallback = () =>
        {
            if (Dialog != null) Dialog.Hide();
            Save();
        };

        if (Email.Length < 10)
        {
            Dialog = new ModalDialogViewModel("This email seems a bit too short, are you sure you want to continue saving?",
                                            ModalDialogViewModel.DialogButtons.Ok,
                                            ModalDialogViewModel.CreateCommands(new Action[] { OkCallback }));
            Dialog.Show();
            return;
        }

        if (LastName.Length < 2)
        {

            Dialog = new ModalDialogViewModel("The Lastname seems short. Are you sure that you want to save this Customer?",
                                              ModalDialogViewModel.CreateButtons(ModalDialogViewModel.DialogMode.TwoButton,
                                                                                 new string[] {"Of Course!", "NoWay!"},
                                                                                 OkCallback,
                                                                                 () => Dialog.Hide()));

            Dialog.Show();
            return;
        }

        Save(); // if we got here we can save directly
    }

Here is the code:

ModalDialogView XAML:

    <UserControl x:Class="DemoApp.View.ModalDialog"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Name="root">
        <UserControl.Resources>
            <ResourceDictionary Source="../MainWindowResources.xaml" />
        </UserControl.Resources>
        <Grid>
            <Border Background="#90000000" Visibility="{Binding Visibility}">
                <Border BorderBrush="Black" BorderThickness="1" Background="AliceBlue" 
                        CornerRadius="10,0,10,0" VerticalAlignment="Center"
                        HorizontalAlignment="Center">
                    <Border.BitmapEffect>
                        <DropShadowBitmapEffect Color="Black" Opacity="0.5" Direction="270" ShadowDepth="0.7" />
                    </Border.BitmapEffect>
                    <Grid Margin="10">
                        <Grid.RowDefinitions>
                            <RowDefinition />
                            <RowDefinition />
                            <RowDefinition Height="Auto" />
                        </Grid.RowDefinitions>
                        <TextBlock Style="{StaticResource ModalDialogHeader}" Text="{Binding DialogHeader}" Grid.Row="0"/>
                        <TextBlock Text="{Binding DialogMessage}" Grid.Row="1" TextWrapping="Wrap" Margin="5" />
                        <StackPanel HorizontalAlignment="Stretch" VerticalAlignment="Bottom" Grid.Row="2">
                            <ContentControl HorizontalAlignment="Stretch"
                              DataContext="{Binding Commands}"
                              Content="{Binding}"
                              ContentTemplate="{StaticResource ButtonCommandsTemplate}"
                              />
                        </StackPanel>
                    </Grid>
                </Border>
            </Border>
        </Grid>

    </UserControl>

ModalDialogView code behind:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

namespace DemoApp.View
{
    /// <summary>
    /// Interaction logic for ModalDialog.xaml
    /// </summary>
    public partial class ModalDialog : UserControl
    {
        public ModalDialog()
        {
            InitializeComponent();
            Visibility = Visibility.Hidden;
        }

        private bool _parentWasEnabled = true;

        public bool IsShown
        {
            get { return (bool)GetValue(IsShownProperty); }
            set { SetValue(IsShownProperty, value); }
        }

        // Using a DependencyProperty as the backing store for IsShown.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty IsShownProperty =
            DependencyProperty.Register("IsShown", typeof(bool), typeof(ModalDialog), new UIPropertyMetadata(false, IsShownChangedCallback));

        public static void IsShownChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if ((bool)e.NewValue == true)
            {
                ModalDialog dlg = (ModalDialog)d;
                dlg.Show();
            }
            else
            {
                ModalDialog dlg = (ModalDialog)d;
                dlg.Hide();
            }
        }

        #region OverlayOn

        public UIElement OverlayOn
        {
            get { return (UIElement)GetValue(OverlayOnProperty); }
            set { SetValue(OverlayOnProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Parent.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty OverlayOnProperty =
            DependencyProperty.Register("OverlayOn", typeof(UIElement), typeof(ModalDialog), new UIPropertyMetadata(null));

        #endregion

        public void Show()
        {

            // Force recalculate binding since Show can be called before binding are calculated            
            BindingExpression expressionOverlayParent = this.GetBindingExpression(OverlayOnProperty);
            if (expressionOverlayParent != null)
            {
                expressionOverlayParent.UpdateTarget();
            }

            if (OverlayOn == null)
            {
                throw new InvalidOperationException("Required properties are not bound to the model.");
            }

            Visibility = System.Windows.Visibility.Visible;

            _parentWasEnabled = OverlayOn.IsEnabled;
            OverlayOn.IsEnabled = false;           

        }

        private void Hide()
        {
            Visibility = Visibility.Hidden;
            OverlayOn.IsEnabled = _parentWasEnabled;
        }

    }
}

ModalDialogViewModel:

using System;
using System.Windows.Input;
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.Windows;
using System.Linq;

namespace DemoApp.ViewModel
{

    /// <summary>
    /// Represents an actionable item displayed by a View (DialogView).
    /// </summary>
    public class ModalDialogViewModel : ViewModelBase
    {

        #region Nested types

        /// <summary>
        /// Nested enum symbolizing the types of default buttons used in the dialog -> you can localize those with Localize(DialogMode, string[])
        /// </summary>
        public enum DialogMode
        {
            /// <summary>
            /// Single button in the View (default: OK)
            /// </summary>
            OneButton = 1,
            /// <summary>
            /// Two buttons in the View (default: YesNo)
            /// </summary>
            TwoButton,
            /// <summary>
            /// Three buttons in the View (default: AbortRetryIgnore)
            /// </summary>
            TreeButton,
            /// <summary>
            /// Four buttons in the View (no default translations, use Translate)
            /// </summary>
            FourButton,
            /// <summary>
            /// Five buttons in the View (no default translations, use Translate)
            /// </summary>
            FiveButton
        }

        /// <summary>
        /// Provides some default button combinations
        /// </summary>
        public enum DialogButtons
        {
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration Ok
            /// </summary>
            Ok,
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration OkCancel
            /// </summary>
            OkCancel,
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration YesNo
            /// </summary>
            YesNo,
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration YesNoCancel
            /// </summary>
            YesNoCancel,
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration AbortRetryIgnore
            /// </summary>
            AbortRetryIgnore,
            /// <summary>
            /// As System.Window.Forms.MessageBoxButtons Enumeration RetryCancel
            /// </summary>
            RetryCancel
        }

        #endregion

        #region Members

        private static Dictionary<DialogMode, string[]> _translations = null;

        private bool _dialogShown;
        private ReadOnlyCollection<CommandViewModel> _commands;
        private string _dialogMessage;
        private string _dialogHeader;

        #endregion

        #region Class static methods and constructor

        /// <summary>
        /// Creates a dictionary symbolizing buttons for given dialog mode and buttons names with actions to berform on each
        /// </summary>
        /// <param name="mode">Mode that tells how many buttons are in the dialog</param>
        /// <param name="names">Names of buttons in sequential order</param>
        /// <param name="callbacks">Callbacks for given buttons</param>
        /// <returns></returns>
        public static Dictionary<string, Action> CreateButtons(DialogMode mode, string[] names, params Action[] callbacks) 
        {
            int modeNumButtons = (int)mode;

            if (names.Length != modeNumButtons)
                throw new ArgumentException("The selected mode needs a different number of button names", "names");

            if (callbacks.Length != modeNumButtons)
                throw new ArgumentException("The selected mode needs a different number of callbacks", "callbacks");

            Dictionary<string, Action> buttons = new Dictionary<string, Action>();

            for (int i = 0; i < names.Length; i++)
            {
                buttons.Add(names[i], callbacks[i]);
            }

            return buttons;
        }

        /// <summary>
        /// Static contructor for all DialogViewModels, runs once
        /// </summary>
        static ModalDialogViewModel()
        {
            InitTranslations();
        }

        /// <summary>
        /// Fills the default translations for all modes that we support (use only from static constructor (not thread safe per se))
        /// </summary>
        private static void InitTranslations()
        {
            _translations = new Dictionary<DialogMode, string[]>();

            foreach (DialogMode mode in Enum.GetValues(typeof(DialogMode)))
            {
                _translations.Add(mode, GetDefaultTranslations(mode));
            }
        }

        /// <summary>
        /// Creates Commands for given enumeration of Actions
        /// </summary>
        /// <param name="actions">Actions to create commands from</param>
        /// <returns>Array of commands for given actions</returns>
        public static ICommand[] CreateCommands(IEnumerable<Action> actions)
        {
            List<ICommand> commands = new List<ICommand>();

            Action[] actionArray = actions.ToArray();

            foreach (var action in actionArray)
            {
                //RelayExecuteWrapper rxw = new RelayExecuteWrapper(action);
                Action act = action;
                commands.Add(new RelayCommand(x => act()));
            }

            return commands.ToArray();
        }

        /// <summary>
        /// Creates string for some predefined buttons (English)
        /// </summary>
        /// <param name="buttons">DialogButtons enumeration value</param>
        /// <returns>String array for desired buttons</returns>
        public static string[] GetButtonDefaultStrings(DialogButtons buttons)
        {
            switch (buttons)
            {
                case DialogButtons.Ok:
                    return new string[] { "Ok" };
                case DialogButtons.OkCancel:
                    return new string[] { "Ok", "Cancel" };
                case DialogButtons.YesNo:
                    return new string[] { "Yes", "No" };
                case DialogButtons.YesNoCancel:
                    return new string[] { "Yes", "No", "Cancel" };
                case DialogButtons.RetryCancel:
                    return new string[] { "Retry", "Cancel" };
                case DialogButtons.AbortRetryIgnore:
                    return new string[] { "Abort", "Retry", "Ignore" };
                default:
                    throw new InvalidOperationException("There are no default string translations for this button configuration.");
            }
        }

        private static string[] GetDefaultTranslations(DialogMode mode)
        {
            string[] translated = null;

            switch (mode)
            {
                case DialogMode.OneButton:
                    translated = GetButtonDefaultStrings(DialogButtons.Ok);
                    break;
                case DialogMode.TwoButton:
                    translated = GetButtonDefaultStrings(DialogButtons.YesNo);
                    break;
                case DialogMode.TreeButton:
                    translated = GetButtonDefaultStrings(DialogButtons.YesNoCancel);
                    break;
                default:
                    translated = null; // you should use Translate() for this combination (ie. there is no default for four or more buttons)
                    break;
            }

            return translated;
        }

        /// <summary>
        /// Translates all the Dialogs with specified mode
        /// </summary>
        /// <param name="mode">Dialog mode/type</param>
        /// <param name="translations">Array of translations matching the buttons in the mode</param>
        public static void Translate(DialogMode mode, string[] translations)
        {
            lock (_translations)
            {
                if (translations.Length != (int)mode)
                    throw new ArgumentException("Wrong number of translations for selected mode");

                if (_translations.ContainsKey(mode))
                {
                    _translations.Remove(mode);
                }

                _translations.Add(mode, translations);

            }
        }

        #endregion

        #region Constructors and initialization

        public ModalDialogViewModel(string message, DialogMode mode, params ICommand[] commands)
        {
            Init(message, Application.Current.MainWindow.GetType().Assembly.GetName().Name, _translations[mode], commands);
        }

        public ModalDialogViewModel(string message, DialogMode mode, params Action[] callbacks)
        {
            Init(message, Application.Current.MainWindow.GetType().Assembly.GetName().Name, _translations[mode], CreateCommands(callbacks));
        }

        public ModalDialogViewModel(string message, Dictionary<string, Action> buttons)
        {
            Init(message, Application.Current.MainWindow.GetType().Assembly.GetName().Name, buttons.Keys.ToArray(), CreateCommands(buttons.Values.ToArray()));
        }

        public ModalDialogViewModel(string message, string header, Dictionary<string, Action> buttons)
        {
            if (buttons == null)
                throw new ArgumentNullException("buttons");

            ICommand[] commands = CreateCommands(buttons.Values.ToArray<Action>());

            Init(message, header, buttons.Keys.ToArray<string>(), commands);
        }

        public ModalDialogViewModel(string message, DialogButtons buttons, params ICommand[] commands)
        {
            Init(message, Application.Current.MainWindow.GetType().Assembly.GetName().Name, ModalDialogViewModel.GetButtonDefaultStrings(buttons), commands);
        }

        public ModalDialogViewModel(string message, string header, DialogButtons buttons, params ICommand[] commands)
        {
            Init(message, header, ModalDialogViewModel.GetButtonDefaultStrings(buttons), commands);
        }

        public ModalDialogViewModel(string message, string header, string[] buttons, params ICommand[] commands)
        {
            Init(message, header, buttons, commands);
        }

        private void Init(string message, string header, string[] buttons, ICommand[] commands)
        {
            if (message == null)
                throw new ArgumentNullException("message");

            if (buttons.Length != commands.Length)
                throw new ArgumentException("Same number of buttons and commands expected");

            base.DisplayName = "ModalDialog";
            this.DialogMessage = message;
            this.DialogHeader = header;

            List<CommandViewModel> commandModels = new List<CommandViewModel>();

            // create commands viewmodel for buttons in the view
            for (int i = 0; i < buttons.Length; i++)
            {
                commandModels.Add(new CommandViewModel(buttons[i], commands[i]));
            }

            this.Commands = new ReadOnlyCollection<CommandViewModel>(commandModels);

        }

        #endregion

                                                                                                                                                                                                                                                            #region Properties

    /// <summary>
    /// Checks if the dialog is visible, use Show() Hide() methods to set this
    /// </summary>
    public bool DialogShown
    {
        get
        {
            return _dialogShown;
        }
        private set
        {
            _dialogShown = value;
            base.OnPropertyChanged("DialogShown");
        }
    }

    /// <summary>
    /// The message shown in the dialog
    /// </summary>
    public string DialogMessage
    {
        get
        {
            return _dialogMessage;
        }
        private set
        {
            _dialogMessage = value;
            base.OnPropertyChanged("DialogMessage");
        }
    }

    /// <summary>
    /// The header (title) of the dialog
    /// </summary>
    public string DialogHeader
    {
        get
        {
            return _dialogHeader;
        }
        private set
        {
            _dialogHeader = value;
            base.OnPropertyChanged("DialogHeader");
        }
    }

    /// <summary>
    /// Commands this dialog calls (the models that it binds to)
    /// </summary>
    public ReadOnlyCollection<CommandViewModel> Commands
    {
        get
        {
            return _commands;
        }
        private set
        {
            _commands = value;
            base.OnPropertyChanged("Commands");
        }
    }

    #endregion

        #region Methods

        public void Show()
        {
            this.DialogShown = true;
        }

        public void Hide()
        {
            this._dialogMessage = String.Empty;
            this.DialogShown = false;
        }

        #endregion
    }
}

ViewModelBase has :

public virtual string DisplayName { get; protected set; }

and implements INotifyPropertyChanged

Some resources to put in the resource dictionary:

<!--
This style gives look to the dialog head (used in the modal dialog)
-->
<Style x:Key="ModalDialogHeader" TargetType="{x:Type TextBlock}">
    <Setter Property="Background" Value="{StaticResource Brush_HeaderBackground}" />
    <Setter Property="Foreground" Value="White" />
    <Setter Property="Padding" Value="4" />
    <Setter Property="HorizontalAlignment" Value="Stretch" />
    <Setter Property="Margin" Value="5" />
    <Setter Property="TextWrapping" Value="NoWrap" />
</Style>

<!--
This template explains how to render the list of commands as buttons (used in the modal dialog)
-->
<DataTemplate x:Key="ButtonCommandsTemplate">
    <ItemsControl IsTabStop="False" ItemsSource="{Binding}" Margin="6,2">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Button MinWidth="75" Command="{Binding Path=Command}" Margin="4" HorizontalAlignment="Right">
                    <TextBlock Text="{Binding Path=DisplayName}" Margin="2"></TextBlock>
                </Button>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" />
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</DataTemplate>

Passed

One issue I've run into with this sort of pseudo dialog is that a user could still tab right out of the control into the ones in the background, where the user could modify other controls with the keyboard. Ultimately resolved by capturing some event like LostKeyboardFocus and programatically setting focus back to the pseudo-dialog. – Caruncle

@Caruncle it seems you have not set the binding OverlayOn of the dialog to the container of all other controls in the View. The dialog disables that control when it gets visible. And if that control contains all other controls in the view you should not be able to tab to the. You would be able to tab out to some other parent view though. See the section "XAML Usage in another view". – Bullington

Is there any chance you can post a full sample, or correct this one, as I'm having trouble with various elements being incorrect or missing. The Visibility binding for example, on the modal dialog xaml, has no property to bind to on the viewmodel, it's datacontext. – Conjecture

Answer 2

I have a custom open source FrameworkElement on my GitHub page that allows you to display modal content over the primary content.

The control can be used like this:

<c:ModalContentPresenter IsModal="{Binding DialogIsVisible}">
    <TabControl Margin="5">
            <Button Margin="55"
                    Padding="10"
                    Command="{Binding ShowModalContentCommand}">
                This is the primary Content
            </Button>
        </TabItem>
    </TabControl>

    <c:ModalContentPresenter.ModalContent>
        <Button Margin="75"
                Padding="50"
                Command="{Binding HideModalContentCommand}">
            This is the modal content
        </Button>
    </c:ModalContentPresenter.ModalContent>

</c:ModalContentPresenter>

Features:

• Displays arbitrary content.
• Does not disable the primary content whilst the modal content is being displayed.
• Disables mouse and keyboard access to the primary content whilst the modal content is displayed.
• Is only modal to the content it is covering, not the entire application.
• can be used in an MVVM friendly way by binding to the IsModal property.

Cheiron

Answer 3

I would approach this as a service that gets injected into your ViewModel, along the lines of the sample code below. To the extent what you want to do is in fact message box behavior, I would have my service implementation use a MessageBox!

I am using KISS here in order to present the concept. No code behind, and completely unit testable as shown.

As an aside, that Josh Smith example you are working off of was incredibly helpful to me also, even if it doesn't cover everything

HTH,
Berry

/// <summary>
/// Simple interface for visually confirming a question to the user
/// </summary>
public interface IConfirmer
{
    bool Confirm(string message, string caption);
}

public class WPFMessageBoxConfirmer : IConfirmer
{
    #region Implementation of IConfirmer

    public bool Confirm(string message, string caption) {
        return MessageBox.Show(message, caption, MessageBoxButton.YesNo) == MessageBoxResult.Yes;
    }

    #endregion
}

// SomeViewModel uses an IConfirmer
public class SomeViewModel
{

    public ShellViewModel(ISomeRepository repository, IConfirmer confirmer) 
    {
        if (confirmer == null) throw new ArgumentNullException("confirmer");
        _confirmer = confirmer;

        ...
    }
    ...

    private void _delete()
    {
        var someVm = _masterVm.SelectedItem;
        Check.RequireNotNull(someVm);

        if (detailVm.Model.IsPersistent()) {
            var msg = string.Format(GlobalCommandStrings.ConfirmDeletion, someVm.DisplayName);
            if(_confirmer.Confirm(msg, GlobalCommandStrings.ConfirmDeletionCaption)) {
                _doDelete(someVm);
            }
        }
        else {
            _doDelete(someVm);
        }
    }
    ...
}

// usage in the Production code 
var vm = new SomeViewModel(new WPFMessageBoxConfirmer());

// usage in a unit test
[Test]
public void DeleteCommand_OnExecute_IfUserConfirmsDeletion_RemovesSelectedItemFrom_Workspaces() {
    var confirmerMock = MockRepository.GenerateStub<IConfirmer>();
    confirmerMock.Stub(x => x.Confirm(Arg<string>.Is.Anything, Arg<string>.Is.Anything)).Return(true);
    var vm = new ShellViewModel(_repository, _crudConverter, _masterVm, confirmerMock, _validator);

    vm.EditCommand.Execute(null);
    Assert.That(vm.Workspaces, Has.Member(_masterVm.SelectedItem));
    Assert.That(vm.Workspaces, Is.Not.Empty);

    vm.DeleteCommand.Execute(null);
    Assert.That(vm.Workspaces, Has.No.Member(_masterVm.SelectedItem));
    Assert.That(vm.Workspaces, Is.Empty);
}

Farlay

I see you are using return MessageBox.Show(...); which is a blocking call (ie. the whole window becomes unusable). This is not exactly what I needed. If I needed to block a higher level view from this viewmodel I would simply pass the Dialog from a higher level viewmodel in the constructor. But nevertheless seems an elegant solution. – Bullington

@Marino. Well yeah, but that's kind of the point. I wanted to keep it easy to demonstrate one way to do this in a testable MVVM style. You can vary the implementation detail(s) as needed (ShowModal, ShowDialog), and change the interface to be broader as well (IDialogService, etc). The conceptual mvvm approach would be the same though. Cheers – Farlay