Question

I have the following class hierarchy:

public abstract class BaseData {   //some properties }  public class CoData : BaseData {   //some properties } 

I am working with a method that requires the return type to be List<BaseData>. In the method, I have access to List<CoData>

public List<BaseData> Save() {   List<CoData> listCoData = GetData();   return listCoData; } 

If I understand correctly, I can upcast from a CoData to a BaseData. But, when I have a list, it errors out even if I explicitly try to typecast.

Error:

Error   118 Cannot implicitly convert type 'System.Collections.Generic.List<CoData>' to System.Collections.Generic.List<BaseData>' 

EDIT:

mquander’s Conversion approach seems to work for me in 3.0

Is downcasting done the same way as well? from

ie., Can I do this – List<CoData> listCoData = listBaseData.Cast<BaseData>().ToList();

Answer 1

Yes; welcome to variance. Ultimately, it isn’t a list of BaseData – for example, if you had another subclass, a List<BaseData> would (at compile time) let you .Add it… but the runtime type wouldn’t let you. The compiler is stopping you making a mistake.

In some scenarios, generics can help here… I discuss this at the end of this blog entry. Note that .NET 4.0 variance doesn’t apply to lists.