Question

I need a base class with a property where I can derive classes with the same property but different (compatible) types. The base Class can be abstract.

public class Base
{
    public virtual object prop { get; set; }
}

public class StrBase : Base
{
    public override string prop { get; set; } // compiler error
}

public class UseIt
{
    public void use()
    {
        List<Base> l = new List<Base>();
        //...
    }
}

---

I tried it with Generics but that gives me a problem when using the class, because I want to store differently typed base classes in the List.

public class BaseG<T>
{
    public T prop { get; set; }
}

public class UseIt
{
    public void use()
    {
        List<BaseG> l = new List<BaseG>(); // requires type argument
        //...
    }
}

Answer 1

Here’s an alternative approach to proposed solution:

public abstract class Base
{
    public abstract void Use();
    public abstract object GetProp();
}

public abstract class GenericBase<T> : Base
{
    public T Prop { get; set; }

    public override object GetProp()
    {
        return Prop;
    }
}

public class StrBase : GenericBase<string>
{
    public override void Use()
    {
        Console.WriteLine("Using string: {0}", Prop);
    }
}

public class IntBase : GenericBase<int>
{
    public override void Use()
    {
        Console.WriteLine("Using int: {0}", Prop);
    }
}

Basically I’ve added a generic class in the middle that stores your properly-typed property. this will work assuming that you never need to access Prop from the code that iterates the members of the List<Base>. (You could always add an abstract method to Base called GetProp that casts the generic to an object if that’s required.)

Sample usage:

class Program
{
    static void Main(string[] args)
    {
        List<Base> l = new List<Base>();

        l.Add(new StrBase {Prop = "foo"});
        l.Add(new IntBase {Prop = 42});

        Console.WriteLine("Using each item");
        foreach (var o in l)
        {
            o.Use();
        }
        Console.WriteLine("Done");
        Console.ReadKey();
    }
}

Edit: Added the GetProp() method to illustrate how the property can be directly accessed from the base class.