What parameters should I pass to my ViewModel's constructor?

Question

I'm pretty novice to WPF/MVVM and trying to understand this pattern. I'm exploring this MVVM application http://msdn.microsoft.com/en-us/magazine/dd419663.aspx

Author created Customer class which is stored in Model folder and CustomerRepository class which is stored in DataAccess folder

CustomerRepository contains methods that 'do stuff', for example static List<Customer> LoadCustomers(string customerDataFile) I.e. we can't say that CustomerRepository is pure model file, it's also kind of utility file.

From another hand CustomerRepository stores important data readonly List<Customer> _customers; and we know that we should store such kind of data in model!

Well I would say that CustomerRepository is mixing things - it's a model file that contains some utility methods inside it.

The problem is that CustomerRepository instance is passed to ViewModel public AllCustomersViewModel(CustomerRepository customerRepository). Now viewModel contains something that it shouldn't, in particular it can force CustomerRepository to reload stuff etc.

In my opinion this is agains MVVM pattern, I think that ViewModel should only contain references to model files all utility classes and methods to manipulate model files should be in other places.

Am I right or wrong? Is it ok to pass class that offers some services on model (can reload/refhresh model etc.) to ViewModel? I.e. where should be so-called service layer, is it ok to have it in ViewModel?

Answer 1

You are wrong in your assumption that "ViewModel should only contain references to model files".

ViewModel can reference anything it needs, as long as it isn't a view.

It is very common to inject services into a ViewModel.

Answer 2

AFAIK, there are two widespreas treatments of the MVVM pattern.

The first, which you seem to be more familiar with, includes 4 components: View, Model, ViewModel and Controller. Here, ViewModel contains only UI logic and exposes events to Controller which serves for synchronization between ViewModel and Model.

The second, which is used in the article, simply merges ViewModel and Controller into ViewModel. BTW, Martin Fowler describes this case: http://martinfowler.com/eaaDev/PresentationModel.html

The choice depends on personal preferences as I see it.