C# Collection with class inheritance and generic type inheritance does not allow to add inherited class

Question

I encounter something maybe possible, but does not work.

Here is sample class,

abstract class BaseModel { }

abstract class BaseViewModel<T> where T : BaseModel
{
    protected T model;
}

class ModelA : BaseModel { }

class ViewModelA : BaseViewModel<ModelA> { }

and now I try to do

class Program
{
    static void Main(string[] args)
    {
        var collection = new List<BaseViewModel<BaseModel>>();
        collection.Add(new ViewModelA());
    }
}

Intuitively, it looks to be work. However it does not possible to add ViewModelA.

I want to know why this is not possible, and work around of this situation.

A BaseViewModel<BaseModel> must be able to handle anything that inherits from BaseModel. You are passing it a ViewModelA, which can only handle a subset of things that inherit from BaseModel, i.e. objects that inehrit from ModelA. So the assignment is not allowed. — John Wu
– John Wu, Commented Sep 8, 2020 at 6:04
the workaround is: rethink your design. Is your list really of the base-type or do all your entitites are allways of the derived type? What is your cinsuming code that uses the collection? — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Sep 8, 2020 at 6:17
Imagine this would be possible. Then users of your code could also write collection.Add(AnotherViewModel). — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Sep 8, 2020 at 6:18
@HimBromBeere I am trying to allow collection.Add(AnotherViewModel) with the restriction to inherit BaseViewModel<T> where T : BaseModel. I understand it could be a design problem but I was curious why it does not working. — ibocon
– ibocon, Commented Sep 8, 2020 at 6:29

TheGeneral · Accepted Answer · 2020-09-08 06:33:25Z

ViewModelA is not equal to BaseViewModel<BaseModel> even though you may think it is

This is because classes are invariant.

Invariance : means that you can use only the type originally specified
Covariance : enables you to use a more derived type than originally specified.

One way to achieve what you want (potentially) is to use a covariant interface parameter via the out generic modifier. However, it has certain limitations. Type parameters can only be used as a return type of interface methods and not used as a type of method arguments

Example

public interface IBaseViewModel<out T>  where T : BaseModel 
{ 
   ... 
}

abstract class BaseViewModel<T> : IBaseViewModel<T> where T : BaseModel 
{
   protected T model; 
}

class ModelA : BaseModel { }

class ViewModelA : BaseViewModel<ModelA> { }

static void Main(string[] args)
{
   var collection = new List<IBaseViewModel<BaseModel>>();
   collection.Add(new ViewModelA()); // this now works
}

Purushothaman · Accepted Answer · 2020-09-08 06:41:10Z

0

I have different theory in simple words about your implementation, As per your implementation both your BaseViewModel<T> and BaseModel are abstract which mean you cannot instance an abstract class, meaning you cannot use BaseViewModel<BaseModel> baseModel = new ViewModelA();. When you adding to the collection you are trying to create an instance for the abstract class which violates the abstract class rules. I have modified your a little bit so that you can add to the collection.


abstract class BaseModel { }

abstract class BaseViewModel<T> where T : BaseModel
{
     protected T model;
}

class ModelA : BaseModel { }

class ViewModelA : BaseViewModel<ModelA> { }

static async Task Main(string[] args)
{
    var collection = new List<BaseViewModel<ModelA>>();
    collection.Add(new ViewModelA());
}

You can also make interface as per the previous answers to make decoupled class.

edited Sep 8, 2020 at 6:41

answered Sep 8, 2020 at 6:27

Purushothaman

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2 Comments

ibocon Over a year ago

Thanks you helped me to understand this situation better, however if make collection to List<BaseViewModel<ModelA>>, I can not add ViewModelB : BaseViewModel<ModelB> to collection.

Purushothaman Over a year ago

Obviously you cannot ViewModelB: BaseViewModel<ModelB> to the collection which takes an instance of BaseViewModel<ModelA> as the generic type is binded to ModelA. What you could do is that you can create an interface which is inherited in both ModelA and ModelB and then List<BaseViewModel<YourInterface>>. Then you can add any object which inherits the interface, refer to the previous answer posted by @Michael Randall

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