Igor Kulman

Automatic ViewModels and Services registration for (not only) Caliburn.Micro

· Igor Kulman

My MVVM framework of choice, Caliburn.Micro, provides a simple Dependency Injection container, where you have to register all your ViewModels and Services. This is done in the Bootstraper’s Configure method and may look like this:

container.PerRequest<MainViewModel>():
container.PerRequest<AboutViewModel>();

container.Singleton<SessionService>();
container.RegisterSingleton(typeof(ISettingsService),null,typeof(SettingsService));

where you typically register your ViewModels as per request and services as singletons.

Of course this is done just once, but having to register a ViewModel each time you create a new one can be a nuisance, especially in a large project.

There is a better way that uses reflection. First, you need to create attributes that will represent registration as per request and a s singleton:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class PerRequestAttribute : Attribute
{
}

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class SingletonAttribute : Attribute
{
}

Iterating over all the non-abstract classes in your assemblies using reflection is quite easy, the tricky part is deciding when to register a class as “itself” and when to register it for an interface it implements. My rule of thumbs is that if the class implements exactly one interface, it is one of my services and I register it for that interface, otherwise I register it as itself (ViewModels descendant from Screen implement circa 5 interfaces).

The final registration code looks different for WinRT (Windows 8, Windows 8.1, Windows Phone 8.1 XAML)

// Automatically registers all view models and services
foreach (var type in (typeof(MainViewModel).GetTypeInfo().Assembly.DefinedTypes)
    .Union((typeof(ContactPointsService).GetTypeInfo().Assembly.DefinedTypes)))
{
    if (type.IsClass && !type.IsAbstract)
    {
        // Non-inherited first, inherited second
        if (type.GetCustomAttributes(inherit: false).OfType<SingletonAttribute>().Any())
        {
            container.RegisterSingleton(type.ImplementedInterfaces.Count()==1 ? type.ImplementedInterfaces.First() : type.AsType(), null, type.AsType());
        }
        else if (type.GetCustomAttributes(inherit: false).OfType<PerRequestAttribute>().Any())
        {
            container.RegisterPerRequest(type.ImplementedInterfaces.Count()==1 ? type.ImplementedInterfaces.First() : type.AsType(), null, type.AsType());
        }
        else if (type.GetCustomAttributes(inherit: true).OfType<SingletonAttribute>().Any())
        {
            container.RegisterSingleton(type.ImplementedInterfaces.Count() == 1 ? type.ImplementedInterfaces.First() : type.AsType(), null, type.AsType());
        }
        else if (type.GetCustomAttributes(inherit: true).OfType<PerRequestAttribute>().Any())
        {
            container.RegisterPerRequest(type.ImplementedInterfaces.Count() == 1 ? type.ImplementedInterfaces.First() : type.AsType(), null, type.AsType());
        }                     
    }
}

and for “classic” .NET (Windows Phone 8)

// Automatically registers all types from the current assembly that have correct attribute
foreach (var type in (Assembly.GetExecutingAssembly().GetTypes())
  .Union(Assembly.Load("business logic assembly name").GetTypes()))
{
    if (type.IsClass && !type.IsAbstract)
    {
        var interfaces = type.GetInterfaces();

        // Non-inherited first, inherited second
        if (type.GetCustomAttributes(inherit: false).OfType<SingletonAttribute>().Any())
        {
            container.RegisterSingleton(interfaces.Count() == 1 ? interfaces.First() : type, null, type);
        }
        else if (type.GetCustomAttributes(inherit: false).OfType<PerRequestAttribute>().Any())
        {
            container.RegisterPerRequest(interfaces.Count()==1 ? interfaces.First() : type, null, type);
        }
        else if (type.GetCustomAttributes(inherit: true).OfType<SingletonAttribute>().Any())
        {
            container.RegisterSingleton(interfaces.Count() == 1 ? interfaces.First() : type, null, type);
        }
        else if (type.GetCustomAttributes(inherit: true).OfType<PerRequestAttribute>().Any())
        {
            container.RegisterPerRequest(interfaces.Count() == 1 ? interfaces.First() : type, null, type);
        }
    }
}

because of reflection differences in WinRT. Do not forget to add the PerRequest or Singleton attribute to your classes to make it work.

See also