Action argument of IList<T> will be bound as Null instead of an empty List<T>

[harshgMSFT]

Today CollectionModelBinder returns false if a value provider cannot provide a value, it is the mutable object binder which creates a model for it.
Since it is the CollectionModelBinder which is responsible for binding collections, it should be the one which creates empty object and not let it slip for other model binders.

[yishaigalatzer]

What are the side effects here, and how is it different from MVC 5/Web API?

[harshgMSFT]

The problem is since today CollectionModelBinder returns false, the MutableObjectBinder takes over and creates empty collection, going forward since the mutable object binder has to deal with object creation based on IBinderMetadata (to support uber binding), it will now in addition to creating top level object will try to bind inner properties and will have all sorts of problems.
Even design wise for collection models CollectionModelBinder should return true and the model binding process should stop there.

is it different from MVC 5/Web API?
Web api and MVC did not create the object if there was no value.
Mvc 6 today does create an empty collection.