Opening save file dialog in MVVM using OnPropertyChange is Ok or Not

Question

I am developing painting application in which i need to save my painting. To save i need to show save file dialog , As i am implementing MVVM pattern i can't directly use event handler. But while implementing i thought of using PropertyChanged event directoly.

I have implemented INotifyPropertyChanged in ViewModel , I have bind all commands. In save command in ViewModel i have called

    OnPropertyChanged("Show Save Dialog"); // in ViewModel

and in code behind of user control I have added event handler as

    ViewModel.PropertyChanged += new // in code behind of user control
          System.ComponentModel.PropertyChangedEventHandler(ViewModel_PropertyChanged);

and in ViewModel_PropertyChanged i have

   switch (e.PropertyName ) // in code behind of user control
        {
            case "Show Save Dialog": ShowSaveFileDialog();// this function shows dialog.
            break;
         }

This works fine in my situation but I don't know the dark side of this implementation.

Is it right ????

score 3 · Accepted Answer · answered Aug 09 '11 at 14:18

Right? There is no right or wrong, only the best choice at the current time. The only way for purists would be to abstract away the process of gaining such input from the user behind an interface, then create a View class which serves this interface and inject it somehow (IoC/DI, or perhaps by composition in the xaml if your ViewModels are instantiated that way).

Personally, I wouldn't spend too much time worrying about this, unless you must worry about this. I've done it both ways. For your average MVVM app, I think it is no great crime to just use a MessageBox, OpenFileDialog, etc. For heavily tested apps, you'll need to abstract it away. And there are other situations that demand it as well. For example, I have code that exists in an application and in a Visual Studio extension. VS has its own dialog types which should be used instead of, say, MessageBox. So abstraction is appropriate. But I wouldn't invest the work unless I had a reason to.

score 1 · Answer 2 · answered Aug 09 '11 at 14:00

Why don't you just create a custom event? Something like this in your ViewModel:

public event EventHandler<EventArgs> ShowSaveDialog;

and then use

ViewModel.ShowSaveDialog += OnShowSaveDialog;

private void OnShowSaveDialog(object sender, EventArgs e){
    //handle the event
}

I wouldn't just "abuse" PropertyChanged like that. There's probably nothing wrong with your implementation, it just doesn't feel right. Also, you're using magic strings, but if you have a custom event it's much more declarative, and other users of your code will immediately figure out there is a way to subscribe to this event.
If you need to pass extra information, then make a implementation of EventArgs and add the properties you need.

score 1 · Answer 3 · edited May 23 '17 at 11:58

Well as Will said there is no right or wrong ... only best choices!

Personally, I tend more to the purist side and try to architect a system to have a clear separation of concerns ... but that's just me!

Here is an answer I gave to a post dealing with the same problem as yours. There I show the two current approaches floating around when you want to keep your view model clean of any view code.

But again, choose the way that best fits your needs/preferences!

Opening save file dialog in MVVM using OnPropertyChange is Ok or Not

3 Answers3