Question

We have a problem with the usage of generics.
We have a generic collection of generic keyvalue pair which is defined as follows

public class KeyValueTemplate<K, V> : IGetIdentifier<K>
{
//...
}

public class KeyValueListTemplate<K, V> : ObservableCollection<KeyValueTemplate<K, V>>
{
//....
}

public class KeyValueStringListTemplate : KeyValueListTemplate<string,string> { }

We are using this in the code as follows

public class test
{

   public KeyValueStringListTemplate SetValuesList{get;set;}
   public ObservableCollection<IGetIdentifier<string>> GetList()
   {
     return SetValuesList;
   }

}

The complier is not accepting this. The error is

Cannot convert type 'KeyValueStringListTemplate' to 'System.Collections.ObjectModel.ObservableCollection<IGetIdentifier<string>>

Why?? Both the types are same to me.

Answer 1

This line

public class KeyValueListTemplate<K, V> : ObservableCollection<KeyValueTemplate<K, V>>

defines a new type, KeyValueListTemplate, that is a subtype of ObservableCollection, so they are different types. KeyValueListTemplatecan be safely converted to ObservableCollection, because it has a superset of ObservableCollection‘s functionality (by Liskov Substitution Principle), but the opposite conversion is not safe.