How to instantiate a generic class with an abstract constraint in C#

Question

I have the following classes:

public abstract class ThingBase { }

public class ThingA : ThingBase { }

And the following generic class:

public class ThingOwner<ThingType> where ThingType : ThingBase { }

I would like to create a ThingOwner instance like below:

ThingOwner<ThingBase> thingOwner = new ThingOwner<ThingA>();

With this code, I get the following error: "Cannot implicitly convert type 'ThingOwner(ThingA)' to 'ThingOwner(ThingBase)'" .

I can't figure how to make it work. I know there are lots of existing discussions about generic classes and inheritance but I tried pretty much everything and I couldn't find a solution that worked for me.

Thanks!

Answer 1

You should make use of covariance for generic types introduced in C# 4.0. For this to work you need to use an interface instead of a class:

public interface IThingOwner<out ThingType> where ThingType : ThingBase { }

public class ThingOwner<ThingType> : IThingOwner<ThingType>
    where ThingType : ThingBase
{

}


IThingOwner<ThingBase> thingOwner = new ThingOwner<ThingA>();

Answer 2

Covariance/contravariance is only supported for interfaces. If you need classes, then only these can work:

ThingOwner<ThingBase> thingOwner = new ThingOwner<ThingBase>();
ThingOwner<ThingA> thingOwner = new ThingOwner<ThingA>();

Answer 3

In addition to the above answers some explanation. While your question can be understood, think of the following:

State you have a derived class that accepts a type parameter ClassA . In the ThingOwner<ClassA> it is only allowed to add an instance of a class that is or derives from ClassA . When you cast that to ThingOwner<BaseClass> it is suddenly allowed to add an instance of ClassB which also derived from BaseClass . This can harm your program and is actually wrong. This was why they invented generics in the first place.