ViewModel Locator

[Kevin Marois]

As part of my own framework I am developing Service and View Model Locators. Both are done, but I have one problem. Here's what I have so far: Interface

public interface IViewModelLocator
{
object Get();

void Register(Type viewType, object viewModel);

}

View Model Locator Class

public class ViewModelLocator : IViewModelLocator
{
// Internal list of View Models
private IDictionary _storage;

public ViewModelLocator()
{
    \_storage = new Dictionary<Type, object>();
}

public object Get<T>()
{
    if (\_storage.ContainsKey(typeof(T)))
    {
        return \_storage.FirstOrDefault(x => x.Key == typeof(T)).Value;
    }
    else
    {
        throw new Exception($"The requested view '{typeof(T)}' is not registered");
    }
}

public void Register(Type viewType, object viewModel)
{
    if (!\_storage.ContainsKey(viewType))
    {
        \_storage.Add(viewType, viewModel);
    }
    else
    {
        throw new Exception($"An instance of the view model '{viewModel}' is already registered for the view '{viewType}'");
    }
}

}

App Setup

public partial class App : Application
{
public static IServiceLocator ServiceLocator { get; private set; }
public static IViewModelLocator ViewModelLocator { get; private set; }

public App()
{
    ServiceLocator = new ServiceLocator();
    ServiceLocator.Register(typeof(IMyService), new MyService());

    ViewModelLocator = new ViewModelLocator();
    ViewModelLocator.Register(typeof(MainWindowView), new MainWindowViewModel(ServiceLocator.Get<IMyService>()));
    ViewModelLocator.Register(typeof(LoginView), new LoginViewModel());
}

}

Window Code Behind

public partial class MainWindowView : Window
{
public MainWindowView()
{
InitializeComponent();

    this.DataContext = App.ViewModelLocator.Get<MainWindowView>();
}

}

Problem Everything to this point works fine, except that I don't want to have to call the VM locator in code in the code behind. I would like to use it in XAML. I have seen other approaches where they do this:

x:Class="My.MainView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
DataContext="{Binding Source={StaticResource MyViewModelLocator}, Path=Main}"&g

[Super Lloyd]

You can't reference generic class in Xaml... However I think you can do something! :) How about something like that (pseudo code that might be inspiring)?

DataContext="{Binding Converter={x:Static TestApp:App.ViewModelConverter}, ConvertParameter={x:Type local:MyVMType}}"

Use a Converter with a ConverterParameter? ;)

A new .NET Serializer All in one Menu-Ribbon Bar Taking over the world since 1371!

[Richard Deeming]

Kevin Marois wrote:

public object Get<T>()
{
if (_storage.ContainsKey(typeof(T)))
{
return _storage.FirstOrDefault(x => x.Key == typeof(T)).Value;
}
else
{
throw new Exception($"The requested view '{typeof(T)}' is not registered");
}
}

That's a very strange way of using a dictionary. The alternative is simpler and significantly more efficient:

public object Get<T>()
{
if (!_storage.TryGetValue(typeof(T), out var result)) return result;
throw new ArgumentException($"The requested view '{typeof(T)}' is not registered");
}

And the method doesn't have to be generic - you can have a non-generic overload as well:

public object Get(Type viewType)
{
if (viewType is null) throw new ArgumentNullException(nameof(viewType));
if (!_storage.TryGetValue(viewType, out var result)) return result;
throw new ArgumentException($"The requested view '{viewType}' is not registered", nameof(viewType));
}

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[Richard Deeming]

How about a custom markup extension[^]?

public class LocateViewModelExtension : MarkupExtension
{
public Type ViewType { get; set; }

private Type ResolveViewType(IServiceProvider serviceProvider)
{
    if (ViewType != null) return ViewType;
    
    var rootObjectProvider = (IRootObjectProvider)serviceProvider.GetService(typeof(IRootObjectProvider));
    return rootObjectProvider?.RootObject?.GetType();
}

public override object ProvideValue(IServiceProvider serviceProvider)
{
    Type viewType = ResolveViewType(serviceProvider);
    if (viewType is null) return null;
    if (App.ViewModelLocator is null) return null;
    return App.ViewModelLocator.Get(viewType);
}

}

Usage:

DataContext="{LocateViewModel ViewType={x:Type my:MainView}}"

or simply:

DataContext="{LocateViewModel}"

If you don't specify a ViewType, it should be able to use the type of the root object for the XAML file it's used in.

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[Kevin Marois]

Thank you for both your answers. Working great now.

If it's not broken, fix it until it is. Everything makes sense in someone's mind. Ya can't fix stupid.