TypeNameCache does not handle type duplicates

[gpomykala]

Scenario:

1. assembly A defines type X
2. assembly B defines type X (same FullName)
3. both A and B are being registered in a bootstrapper and added to AssemblySource

Result: boostrapping fails due to type duplicates being added to TypeNameCache

Although it may not be the best practice to use same types from multiple assemblies within an app it does happen and TypeNameCache should not rely on Type.FullName only as it is not uniquely identtifying information (FullName + assembly is). It would not be too hard to define aggregated dictionary key consisting of assembly name + type name

AssemblySource.Instance.CollectionChanged += (s, e) => { switch (e.Action) { case NotifyCollectionChangedAction.Add: e.NewItems.OfType<Assembly>() .SelectMany(a => ExtractTypes(a)) .Apply(t => TypeNameCache.Add(t.FullName, t)); break; case NotifyCollectionChangedAction.Remove: case NotifyCollectionChangedAction.Replace: case NotifyCollectionChangedAction.Reset: TypeNameCache.Clear(); AssemblySource.Instance .SelectMany(a => ExtractTypes(a)) .Apply(t => TypeNameCache.Add(t.FullName, t)); break; } };

[popcatalin81]

@gpomykala there is an Type.AssemblyQualifiedName

However, this would create conflicts, when looking up a view for PersonViewModel which of the following should be chosen?

AppNamespace.Views.PersonView, Lib1, Version=1.0.0.0, Culture=neutral, PublikeyToken=5c5619...
AppNamespace.Views.PersonView, Lib2, Version=1.0.0.0, Culture=neutral, PublikeyToken=13cefg...

?

[gpomykala]

I am evaluating an assembly aware cache along with an update LocateTypeForModelType

` static IDictionary<Assembly, IDictionary<string, Type>> _assemblyScopedTypeNameCaches = new Dictionary<Assembly, IDictionary<string, Type>>();

    protected override void Configure()
    {
        base.Configure();

        var assemblies = SelectAssemblies();
        assemblies.Apply(a =>
                                   {
                                       _assemblyScopedTypeNameCaches.TryAdd(a, new Dictionary<string, Type>());

                                       var assemblyScopedCache = _assemblyScopedTypeNameCaches[a];

                                       AssemblySourceCache.ExtractTypes(a).Apply(t => assemblyScopedCache.TryAdd(t.FullName, t));
                                   });

        ViewLocator.LocateTypeForModelType = (modelType, displayLocation, context) => {
                                                 var viewTypeName = modelType.FullName;
                                                 var typeAssembly = modelType.Assembly;

                                                 if (View.InDesignMode)
                                                 {
                                                     viewTypeName = ViewLocator.ModifyModelTypeAtDesignTime(viewTypeName);
                                                 }

                                                 viewTypeName = viewTypeName.Substring(
                                                                                       0,
                                                                                       viewTypeName.IndexOf('`') < 0
                                                                                           ? viewTypeName.Length
                                                                                           : viewTypeName.IndexOf('`')
                                                                                      );

                                                 var viewTypeList = ViewLocator.TransformName(viewTypeName, context);
                                                 var viewType = FindTypeByNames(typeAssembly, viewTypeList);

                                                 if (viewType == null)
                                                 {
                                                     Debug.WriteLine("View not found. Searched: {0}.", string.Join(", ", viewTypeList.ToArray()));
                                                 }

                                                 return viewType;
                                             };
    }

    private static Type FindTypeByNames(Assembly typeAssembly, IEnumerable<string> names)
    {

        if (names == null || typeAssembly == null)
        {
            return null;
        }

        var cache = _assemblyScopedTypeNameCaches[typeAssembly];

        var type = names.Select(n => cache.GetValueOrDefault(n)).FirstOrDefault(t => t != null);
        return type;
    }`

[nigel-sampson]

I have some concerns with adding this capability in much the same way @popcatalin81 does.

All of the conventions in CM work off the FullName, if you have a view with the full name of Project.Feature.Views.SampleView and there are two different classes with full name Project.Feature.ViewModels.SampleViewModel which one should it chose to use in a deterministic manner.

[gpomykala]

@nigel-sampson i recognize that the scope of changing it properly for all scenarios supported by the framework is much wider than an example I submitted above.

Answering your question: ideally it should return Project.Feature.ViewModels.SampleViewModel from an assembly Project.Feature.Views.SampleView was defined in.

Just my 2 cents...

[popcatalin81]

@gpomykala Yes that would work from an application developer perspective.

If you're a library author, building a componentized UI library you would want it the other way around, you'd want users to be able to selectively override UI components imported from your library in their applications.

The problem is there's no universal best choice here, so there are only two universal solutions in my opinion:

1. Do not allow duplicate Full names
2. Allow duplicates and build tie breaker mechanisms everywhere in the framework, so the user can choose.

Option 2) Is allot more complicated to support and implement and therefore really needs a compelling use case to make it worthwhile. (One such use case would be reusable View libraries, but this pattern is not really common, even thouhg I've encountered and was pretty neat)

[nigel-sampson]

This is certainly an extreme example, if nothing else I'll probably do an audit to make sure the assembly cache is extensible in the right way such that if someone wants to build a "tie breaking cache" it could be wired in correctly.

[nigel-sampson]

Pushed change to make AssemblySource extensible ad unlock this scenario if needed.