Question

I have those classes, an implementation of active record pattern:

public abstract class RecordCollection<T> : ObservableCollection<T> where T : Record
public abstract class Record : INotifyPropertyChanged

every time i want to define a row of a database (ie a Contact), I create a new record class, like:

public class Contact : Record

and a collection of row of database (ie Contacts) as:

public class ContactCollection : RecordCollection<Contact>

but after that i can’t use generic for contain a record colletcion:

ContactCollection contColl = new ContactCollection(databaseman dbMan);
contColl.Load();

RecordCollection<Record> recordColl = contColl;

it give me “Cannot implicitly convert type ContactCollection to >”. Any help would be appreciated.

Answer 1

This casting is not permitted simply because this could happen:

public class AnotherTypeOfContact: Record {}

Now we could do:

recordColl.Add(new AnotherTpyeOfContact()) //runtime exception as no conversion between AnotherTypeOfContact and Contact

This is what is called covariance and contravariance (read more here) between generic types. For this to work, contColl must implement a “out” only interface, which it doesn’t. By this I mean that for type safety to be preserved, contColl can not allow to add items (more formally, T can only be the return type of a method)

Note that in arrays this is allowed, but its a broken covariance, which is unfortunate: more here