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I am pretty new to MVVM and WPF and I am not completely sure if what I am going to ask now is correct.

I am making a MVVM WPF application. I have a SQL Server database and I am using Entity Framework database-first to generate model classes for me. I have created view model classes and from what I understand from the dozens of tutorials I read today is that I need a ObservableCollection which consists of my view model classes. Is that correct?

The problem is that Entity Framework has already generated a database context for me which contains collections but they are using the model classes and if the above is correct then I will need to make the Entity Framework database context use my view model classes. The question is how.

Do I need to create a new database context class which suits my needs and use that instead or is there a simpler approach which I am missing? Here is the model class that Entity Framework has generated for me:

public partial class Parent
{
    public Parent()
    {
        this.Children = new HashSet<Child>();
    }

    public int ID { get; set; }
    public string Name { get; set; }
    public string PIN { get; set; }
    public string Account { get; set; }
    public string Identity_Card { get; set; }
    public string Address { get; set; }

    public virtual ICollection<Child> Children { get; set; }
}

Thanks in advance. Tell me if I have missed to mention something or add a part of the code and I will do it.

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6
  • loooooooot of text to read. paragraphs help for readability :) Commented Feb 10, 2013 at 21:34
  • I edited it a bit. Hope it is more readable now:) Commented Feb 10, 2013 at 22:29
  • hmm, not sure if I follow, but you only need one database context per database scheme. You have that in place, but you want to share that data with you view model? Is that it? if so, what keeps you from it. By the sounds of it you are on the right track Commented Feb 10, 2013 at 22:34
  • The problem is that the collections in the database context are from type DbSet<Parent> and from the tutorials I saw I think they should be ObservableCollection<ParentViewModel>. Is that correct? I have a feeling that there is something wrong here... As I said before I am pretty new to MVVM so this may be just me and there may be nothing wrong. I just want to be sure I am on the right track ;-) Commented Feb 10, 2013 at 23:33
  • Don't forget that the collections on your ViewModel don't really have to be observable unless you're planning on making changes to them while the view is open. It's not a requirement for doing MVVM :) Commented Feb 11, 2013 at 0:25

3 Answers 3

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Something like this would most likely suffice for what @bas was mentioning, and I would agree that it's the desired way to go.. your viewmodels are not your models.

public class MainViewModel : INotifyPropertyChanged
{
    public MainViewModel(IRepository<Parent> parentRepo, IViewModelFactory factory)
    {
        // you might want to set this up as a fancy async method
        // because.. it looks better and it's easier to read
        Task.Factory
            .StartNew(() => parentRepo.GetAll())
            .ContinueWith(t =>
            {
                // Do error checking and all that boring stuff
                ParentViewModels =
                    new ObservableCollection<ParentViewModel>(
                        t.Result.Select(p => factory.Create<ParentViewModel>(p)));
            }, TaskScheduler.FromCurrentSynchronizationContext());
    }

    private ObservableCollection<ParentViewModel> _parentViewModels;
    public ObservableCollection<ParentViewModel> ParentViewModels
    {
        get
        {
            return _parentViewModels;
        }
        set
        {
            _parentViewModels = value;
            RaisePropertyChanged("ParentViewModels");
        }
    }

    // INotifyPropertyChanged implementation goes here
}

// Super secret sauce viewmodelfactory and repository implementations go here
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1 Comment

Thanks! This is perfect! That is what I was missing. The viewmodel is specific for the view and not the model class. I think I am finally starting to understand how MVVM is designed to be used. Thank you again :-)
1

The problem is that Entity Framework has already generated a database context for me which contains collections but they are using the model classes and if the above is correct then I will need to make the Entity Framework database context use my view model classes. The question is how.

Not sure if I understand what you are saying here, but generated classes are generated classes. You're not supposed to mold them into something else.

Your generated classes can be read by your view model classes. Now I can imagine that feels like duplication, but there's more to consider. First of all your database classes can contain more data than you like to show in your views. So just hiding the data so that you only provide your views the data they need is a perfect strategy. I think security-wise it's also safe to say that you don't want to provide the entities that are directly connected to your database, in your view. But that's probably a different discussion.

So create your view models and fill them with the data provided via your entities.

In any event, do not modify your generated classes in such a way that you need to redo it again when you generate them again :).

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Yes. This feels more like the way I think it should be. Could you provide some sample code on how I need to access the data in the database because I am not quite sure how that should happen. I understand how to access the data by using the context but I am not sure how to do that if my viewmodel doesnt have any connection to it. I am still trying to wrap my head around this whole MVVM concept and I am thankfull to anyone that is helping me :-)
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Just replace the references of the models in the current DataContext with the ones with the models you want to use.

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9 Comments

Yes. I thought about that approach but the problem is that when I regenerate the model classes from the database, for example when I add a new column or table I will have to do that over and over again. It is not that much of a problem. I just think that there may be a better way. :)
Then use the generated models for database access then create mapping functions that can convert the EF models to the View Models and vice versa. You could use a library like AutoMapper to help.
Thanks. I will try that:) Just one more thing: Is there a better way I could use the MVVM pattern with entity framework database-first? Am I missing something? I just have that feeling when there is a better way of doing something... I may be wrong but I just want to make sure ;)
No you are not missing anything. Your use of MVVM is fine. Next time if you might want to consider the Entity Framework code-first approach. That way your models will dictate the database schema and you wont have to keep your models in sync with changes to the database.
The project is still in the very beginning so I can switch to code-first if that would be better. Do you think it is going to be easier?
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