MVVM design: Blocking MessageBox in ViewModel

This question refers to a WPF application based on PRISM 5.0 and the MVVM pattern.

Sometimes when users make decisions, that could have unwanted or negative consequences, it is very common to ask the user, if he really wants to go on and proceed.

For example: One common way, is to ask the user with a messagebox, if he really wants to delete data, that can not be restored after deletion.

The problem is: If I call the MessageBox inside the ViewModel, the ViewModel becomes untestable from the outside.

//BAD!
public class ViewModel
{
    public Boolean Delete()
    {
        //Blocking and therefore untestable in automatic UnitTests
        MsgBoxResult result = MsgBox.Show("Do you really want to delete?"); 

        if (result == yes) {//Do stuff that deletes data here;}

    }
}

One possibility would be, to ask the question in a different private method, that calls the public method

//BETTER, BUT OK?
public class ViewModel
{
    private void OnDeleteAction
    {
        MsgBoxResult result = MsgBox.Show("Do you really want to delete?"); 
        if (result == yes) {Delete();}
    }

    public Boolean Delete()
    {
        //Testable from the outside again, because no blocking question

        //Do stuff that deletes data here
    }

My question: Is is this a good way or is there a more elegant way to ask the user inside a ViewModel? Can you give me a hint or link, what is the best for PRISM 5.0?

I know, that a rule of thumb is, not to use any UI elements in the ViewModel, but I see no alternative to a blocking MessageBox or something else, that blocks the process, before proceeding.

Thank you any hints!

Solution

Prism Interactivity is the way to go here. This allows you to do Confirmations, Notifications, and create custom dialogs that work well with the MVVM pattern. I use them successfully in my Prism applications.

Here are some links to some code in the Prism repo on GitHub: