Return base class from Specification in Ardalis Specification for CosmosDB

Question

In CosmosDB collection I have 2 types of entities which inherit from one base class. I want to return base class from specification and be able to convert that base class object to concrete ones. I don't use EF, I use CosmosDB SDK. I want to have something like that:

class BaseClass
{
   string Id;
}


class ClassA : BaseClass
{
string Status;
}

class ClassB : BaseClass
{
string Amount;
}

public class MyScpec : Specification<BaseClass>
{
    public MyScpec () 

{

Query.Select(x =>
(x is ClassA
             ? new ClassA { Id = x.Id, Status = x.Status }

: new ClassB { Id = x.Id, Amount = x.Amount}
));

}
}

And further in code I want to be able to do like that:

var list = _repository.List(new MySpec());

foreach(var item in list)
{
    if(item is ClassA)    { ...}

}

I tried to return BaseClass:

Query.Select(new BaseClass(){...})

But then I can't convert it to child classes.

Also, I tried to do something like in EF, when we can configure entities

builder.HasBaseType<BaseClass>();

But there is no such a possibility in CosmosDB SDK as far as I know.