Question

I am building a WPF application and using the MVVM for the first time. Overall using MVVM has been very interesting and one of the major benefits is the nice separation between the view and the model classes. It kind of disciplines (at least young developers) to not to mix them together.

We have a scenario where a window needs to be closed on a button click after a confirm message box. Now this can achieved the old-way by handling the button click event and closing the window in the Window class itself. Or we can do it MVVM way by creating a command in ViewModel, call Window to show message box..etc.

I understand what needs to done here, but my question is – is it necessary to use MVVM commands in all cases? Are there exceptions where we should not use commands e.g. simple UI actions? Are we not overusing MVVM here? What exactly will the benefits doing everything the MVVM-way?

Answer 1

Or we can do it MVVM way by creating a command in ViewModel, call Window to show message box..etc.

Let me pick this apart, mainly because IMVHO I see this done wrong all the time – a lot of people try to do too much in the VM. First of all, ask yourself the question:

Is the prompt related to the data or business rules in any way whatsoever?

If it isn’t, i.e. it is simply a “are you really sure?” type prompt, then this SHOULD be done purely within the code behind of the view. The only time the viewmodel needs to have any knowledge or take any action is when it actually has something to do with the viewmodel, in which case you should expose a command from the VM, but the actual window closing is still done from the code behind of the view.

The VM should know nothing about the view that it is bound to, that is one of the purposes of the MVVM pattern. It can expose commands, but it shouldn’t know that a user has interacted with a specific UI element¹, and it shouldn’t directly know that the window is about to be closed. It is okay for the VM to prompt (via a dialog service, which you do have, yeah?) that the current data is unsaved, but it doesn’t know about the window in general because it doesn’t know how its data is presented.

Sometimes you will walk a fine line, and it is easy to over analyze whether something should be done purely from the view, purely from the VM, or as a mixture of both. If you remember the role of the VM, and remember that it is okay to have code behind in the view (provided it is only doing view related stuff and handing VM stuff off to the VM) then 99% of the time you won’t have a problem.

¹ For example, the VM shouldn’t know or care whether the user just clicked a button, hyperlink, or touched a hot spot in an image. The same command can be used to handle any of this.