Handling Data Access Layer Exception in MVVM using Inversion on Control

Question

I'm studding MVVM in C#.

I want to use Inversion of Control (IoC). I use the framework Unity.

I don't understand how to handling exception that could be raised from Data Access Layer.

Here a little easy example I do for study:

-- i have omitted the manage of Model validation (IDataErrorInfo) and Services for ViewModel (ex: DialogService) --

XAML View

<TextBox ...  Text="{Binding Path=Id}" />
<TextBox ...  Text="{Binding Path=Name}"/>

DESIGN APPLICATION

MODEL

{
    public class User : INotifyPropertyChanged
    {
        private int _id;
        public int Id
        {
            get{return _id;}
            set
            {
                _id = value;
                this.OnPropertyChanged("Id");
            }
        }

        private string _name;
        public string Name
        {
            get{return _name;}
            set
            {
                _name = value;
                this.OnPropertyChanged("Name");
            }
        }

        public User(int i, string n)
        {
            _id = i;
            _name = n;
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string name)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }
    }
}

DATA ACCESS LAYER

Interface

public interface IDataAccessLayer
{
    Model.User GetUser();
}

Concrete class

public class ConcreteDataAccessLayer : IDataAccessLayer
{
    public ConcreteDataAccessLayer(){}

    Model.User IDataAccessLayer.GetUser()
    {
        //could throw Exception connecting with data source
    }
}

BUSINESS LAYER

public class BusinessLayer
{
    public BusinessLayer(IDataAccessLayer dataAccessLayer)
    {
        if (dataAccessLayer == null)        
        {
            throw new ArgumentNullException("dataAccessLayer");
        }
        this._dataAccessLayer = dataAccessLayer;
    }

    private IDataAccessLayer _dataAccessLayer;

    private QuestionStak.Model.User _user;

    internal QuestionStak.Model.User User
    {
        get 
        {
            if (_user == null)
                _user = _dataAccessLayer.GetUser();
            return _user; 
        }
    }

}

VIEWMODEL

public class ViewModel : INotifyPropertyChanged
{
    public ViewModel(BusinessLayer bl)
    {
        if (bl == null)
        {
            throw new ArgumentNullException("BusinessLayer");
        }
        _businessLayer = bl;
    }

    private BusinessLayer _businessLayer;


    public int Id
    {
        get
        {
            return _businessLayer.User.Id;
        }
        set
        {
            _businessLayer.User.Id =  value;
        }
    }

    public string Name
    {
        get
        {
            return _businessLayer.User.Name;
        }
        set
        {
            _businessLayer.User.Name =  value;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

APPLICATION

public partial class App : Application
{

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        //Inversion of control
        IUnityContainer container = new UnityContainer();

        //Created as singleton
        container.RegisterType<IDataAccessLayer, ConcreteDataAccessLayer>(new ContainerControlledLifetimeManager());
        container.RegisterType<BusinessLayer, BusinessLayer>(new ContainerControlledLifetimeManager());

        MainWindow win = container.Resolve<MainWindow>();

        win.DataContext = container.Resolve<ViewModel>();
        win.Show();
    }
}

Principles follow

• simple constructors (Unity catch all exception and follow http://blog.ploeh.dk/2011/03/03/InjectionConstructorsshouldbesimple/)
• ViewModel must hide Model structure

So I have a problem that I don't understands how to solve:

• If ConcreteDataAccessLayer can't load data (Ex: server not available) during loading of ViewModel the statement _dataAccessLayer.GetUser() throw the exception and in could not manage it (catch by Unity conteiner)
• If somewhere during the loading I manage the exception, the data binding cause the throw of a null exception because _businessLayer.User is null (unable to load the view)

Please, have someone a clean solution for this problem?

Thanks!

Answer 1

If ConcreteDataAccessLayer can't load data (Ex: server not available) during loading of ViewModel the statement _dataAccessLayer.GetUser() throw the exception and in could not manage it (catch by Unity conteiner)

I'm not sure what you mean by 'in could not manage it (catch by Unity container)', as I would only expect a wrapped Unity exception if there was a problem constructing the types that Unity is resolving. Either way, you would still be able to handle the exception yourself. You would probably want to do this in your presentation layer, so that you could bring up a dialog etc.

If somewhere during the loading I manage the exception, the data binding cause the throw of a null exception because _businessLayer.User is null (unable to load the view)

Yes, if you've handled the error in your business layer, then you would need to guard against the User property being null.

However, I think your approach should be reconsidered. Your IDataAccessLayer and BusinessLayer types seem like big dependencies which is going to minimise the amount of code reuse, as well as making unit testing more difficult.

You should try and minimise the dependencies. If your view model is only interested in users for example, then you could use the repository pattern (well really a data access object pattern) to inject a user repository.

Alternatively if you wish to use rich business objects, then your view models would take your user business object as the dependency. You may then decide to expose the model directly to the view, depending on if you wish to violate the DRY principle (which you are currently doing), or the Law of Demeter.

I would also consider using an MVVM framework if you're using the MVVM design pattern.