C# – Good or bad practice for Dialogs in wpf with MVVM

.netc++modal-dialogmvvmwpf

I lately had the problem of creating add and edit dialogs for my wpf app.

All I want to do in my code was something like this. (I mostly use viewmodel first approach with mvvm)

ViewModel which calls a dialog window:

var result = this.uiDialogService.ShowDialog("Dialogwindow Title", dialogwindowVM);
// Do anything with the dialog result

How does it work?

First, I created a dialog service:

public interface IUIWindowDialogService
{
    bool? ShowDialog(string title, object datacontext);
}

public class WpfUIWindowDialogService : IUIWindowDialogService
{
    public bool? ShowDialog(string title, object datacontext)
    {
        var win = new WindowDialog();
        win.Title = title;
        win.DataContext = datacontext;

        return win.ShowDialog();
    }
}

WindowDialog is a special but simple window. I need it to hold my content:

<Window x:Class="WindowDialog"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    Title="WindowDialog" 
    WindowStyle="SingleBorderWindow" 
    WindowStartupLocation="CenterOwner" SizeToContent="WidthAndHeight">
    <ContentPresenter x:Name="DialogPresenter" Content="{Binding .}">

    </ContentPresenter>
</Window>

A problem with dialogs in wpf is the dialogresult = true can only be achieved in code. That's why I created an interface for my dialogviewmodel to implement it.

public class RequestCloseDialogEventArgs : EventArgs
{
    public bool DialogResult { get; set; }
    public RequestCloseDialogEventArgs(bool dialogresult)
    {
        this.DialogResult = dialogresult;
    }
}

public interface IDialogResultVMHelper
{
    event EventHandler<RequestCloseDialogEventArgs> RequestCloseDialog;
}

Whenever my ViewModel thinks it's time for dialogresult = true, then raise this event.

public partial class DialogWindow : Window
{
    // Note: If the window is closed, it has no DialogResult
    private bool _isClosed = false;

    public DialogWindow()
    {
        InitializeComponent();
        this.DialogPresenter.DataContextChanged += DialogPresenterDataContextChanged;
        this.Closed += DialogWindowClosed;
    }

    void DialogWindowClosed(object sender, EventArgs e)
    {
        this._isClosed = true;
    }

    private void DialogPresenterDataContextChanged(object sender,
                              DependencyPropertyChangedEventArgs e)
    {
        var d = e.NewValue as IDialogResultVMHelper;

        if (d == null)
            return;

        d.RequestCloseDialog += new EventHandler<RequestCloseDialogEventArgs>
                                    (DialogResultTrueEvent).MakeWeak(
                                        eh => d.RequestCloseDialog -= eh;);
    }

    private void DialogResultTrueEvent(object sender, 
                              RequestCloseDialogEventArgs eventargs)
    {
        // Important: Do not set DialogResult for a closed window
        // GC clears windows anyways and with MakeWeak it
        // closes out with IDialogResultVMHelper
        if(_isClosed) return;

        this.DialogResult = eventargs.DialogResult;
    }
 }

Now at least I have to create a DataTemplate in my resource file(app.xaml or something):

<DataTemplate DataType="{x:Type DialogViewModel:EditOrNewAuswahlItemVM}" >
        <DialogView:EditOrNewAuswahlItem/>
</DataTemplate>

Well thats all, I can now call dialogs from my viewmodels:

 var result = this.uiDialogService.ShowDialog("Dialogwindow Title", dialogwindowVM);

Now my question, do you see any problems with this solution?

Edit: for completeness. The ViewModel should implement IDialogResultVMHelper and then it can raise it within a OkCommand or something like this:

public class MyViewmodel : IDialogResultVMHelper
{
    private readonly Lazy<DelegateCommand> _okCommand;

    public MyViewmodel()
    {
         this._okCommand = new Lazy<DelegateCommand>(() => 
             new DelegateCommand(() => 
                 InvokeRequestCloseDialog(
                     new RequestCloseDialogEventArgs(true)), () => 
                         YourConditionsGoesHere = true));
    }

    public ICommand OkCommand
    { 
        get { return this._okCommand.Value; } 
    }

    public event EventHandler<RequestCloseDialogEventArgs> RequestCloseDialog;
    private void InvokeRequestCloseDialog(RequestCloseDialogEventArgs e)
    {
        var handler = RequestCloseDialog;
        if (handler != null) 
            handler(this, e);
    }
 }

EDIT 2: I used the code from here to make my EventHandler register weak:
http://diditwith.net/2007/03/23/SolvingTheProblemWithEventsWeakEventHandlers.aspx
(Website no longer exists, WebArchive Mirror)

public delegate void UnregisterCallback<TE>(EventHandler<TE> eventHandler) 
    where TE : EventArgs;

public interface IWeakEventHandler<TE> 
    where TE : EventArgs
{
    EventHandler<TE> Handler { get; }
}

public class WeakEventHandler<T, TE> : IWeakEventHandler<TE> 
    where T : class 
    where TE : EventArgs
{
    private delegate void OpenEventHandler(T @this, object sender, TE e);

    private readonly WeakReference mTargetRef;
    private readonly OpenEventHandler mOpenHandler;
    private readonly EventHandler<TE> mHandler;
    private UnregisterCallback<TE> mUnregister;

    public WeakEventHandler(EventHandler<TE> eventHandler,
                                UnregisterCallback<TE> unregister)
    {
        mTargetRef = new WeakReference(eventHandler.Target);

        mOpenHandler = (OpenEventHandler)Delegate.CreateDelegate(
                           typeof(OpenEventHandler),null, eventHandler.Method);

        mHandler = Invoke;
        mUnregister = unregister;
    }

    public void Invoke(object sender, TE e)
    {
        T target = (T)mTargetRef.Target;

        if (target != null)
            mOpenHandler.Invoke(target, sender, e);
        else if (mUnregister != null)
        {
            mUnregister(mHandler);
            mUnregister = null;
        }
    }

    public EventHandler<TE> Handler
    {
        get { return mHandler; }
    }

    public static implicit operator EventHandler<TE>(WeakEventHandler<T, TE> weh)
    {
        return weh.mHandler;
    }
}

public static class EventHandlerUtils
{
    public static EventHandler<TE> MakeWeak<TE>(this EventHandler<TE> eventHandler, 
                                                    UnregisterCallback<TE> unregister)
        where TE : EventArgs
    {
        if (eventHandler == null)
            throw new ArgumentNullException("eventHandler");

        if (eventHandler.Method.IsStatic || eventHandler.Target == null)
            throw new ArgumentException("Only instance methods are supported.",
                                            "eventHandler");

        var wehType = typeof(WeakEventHandler<,>).MakeGenericType(
                          eventHandler.Method.DeclaringType, typeof(TE));

        var wehConstructor = wehType.GetConstructor(new Type[] 
                             { 
                                 typeof(EventHandler<TE>), typeof(UnregisterCallback<TE>) 
                             });

        IWeakEventHandler<TE> weh = (IWeakEventHandler<TE>)wehConstructor.Invoke(
                                        new object[] { eventHandler, unregister });

        return weh.Handler;
    }
}

Best Solution

This is a good approach and I used similar ones in the past. Go for it!

One minor thing I'd definitely do is make the event receive a boolean for when you need to set "false" in the DialogResult.

event EventHandler<RequestCloseEventArgs> RequestCloseDialog;

and the EventArgs class:

public class RequestCloseEventArgs : EventArgs
{
    public RequestCloseEventArgs(bool dialogResult)
    {
        this.DialogResult = dialogResult;
    }

    public bool DialogResult { get; private set; }
}
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