prism:ViewModelLocator.AutoWireViewModel="True" will not work for not referenced assemblies

Basically, when the ViewModelLocationProvider calls the _defaultViewTypeToViewModelTypeResolver, the call to Type.GetType(string) returns null.

This might be related to how MEF loads assemblies in general. This seems to be a common probem with MEF and a Google search will return a lot of results with similar issues.Here is someone with the same problem:

Type.GetType returns null when using MEF

You could try adding the plugin location in the probing path of the application.

I personally never use MEF as a DI container, because it's not one. But that is a conversation for another day.

EDIT:Actually, I just thought of a better way to get around this. Simply override ConfigureViewModelLocator in your bootstrapper like this:

        protected override void ConfigureViewModelLocator()    {        base.ConfigureViewModelLocator();        ViewModelLocationProvider.SetDefaultViewTypeToViewModelTypeResolver(viewType =>        {            var viewName = viewType.FullName;            viewName = viewName.Replace(".Views.", ".ViewModels.");            var viewAssemblyName = viewType.GetTypeInfo().Assembly.FullName;            var suffix = viewName.EndsWith("View") ? "Model" : "ViewModel";            var viewModelName = String.Format(CultureInfo.InvariantCulture, "{0}{1}", viewName, suffix);            var assembly = viewType.GetTypeInfo().Assembly;            var type = assembly.GetType(viewModelName, true);            return type;        });    }

This way we can ask the assembly for the type directly and not try to have the framework figure it out for us.