Question

I am learning EF4.1 in conjunction with MVVM and in one of these tutorials they create a MainWindowViewModel with a Repository object that they use in calling another view model (EmployeeListViewModel). Here is the code:

public MainWindowViewModel()
{
    IObjectContextAdapter adapter = ((IObjectContextAdapter)new SidekickEntities());
    _vmRepository = new GenericRepository(adapter.ObjectContext);
    EmployeeListViewModel viewModel = new EmployeeListViewModel(_vmRepository);
    this.ViewModels.Add(viewModel);
}

public EmployeeListViewModel(GenericRepository repository)
{

    if (repository == null)
    {
        throw new ArgumentNullException("repository");
    }

    _employeeRepository = repository;
    this.AllEmployees = new ObservableCollection<Employee>(_employeeRepository.GetAll<Employee>());
}

What I am wondering is, why is the repository created in the MainWindowViewModel and then passed into EmployeeListViewModel? Why not just create the repository in EmployeeListViewModel like this:

public MainWindowViewModel()
{
    EmployeeListViewModel viewModel = new EmployeeListViewModel();
    this.ViewModels.Add(viewModel);
}

public EmployeeListViewModel()
{
    IObjectContextAdapter adapter = ((IObjectContextAdapter)new SidekickEntities());
    _employeeRepository = new GenericRepository(adapter.ObjectContext);
    this.AllEmployees = new ObservableCollection<Employee>(_employeeRepository.GetAll<Employee>());
}

I am pretty new to EF but it seems a little cleaner to create a repository in each separate ViewModel. Wouldn’t the repository then be cleaned up when the ViewModel in question is no longer used instead of keeping it around until the closing of MainWindowViewModel or does this create too many repository instances?

Answer 1

The problem you will run into when using a separate Context for each ViewModel is that entities that have links to each other come from different contexts so you will have to manually work this out when trying to update something (see Attaching and Detaching Objects)

This has to do with a pattern called Unit Of Work.

UoW helps you with encapsulating functions that should work together. In your case the ObjectContext implements the UoW pattern and is passed to the repository in the constructor.

You should share your ObjectContext between functions that work together and should be seen as one Unit Of Work.