Return derived class instance instead of base class instance

Question

I have an interface that looks like this

 public interface SomeInterface<T, U>
    where T : struct
    where U : class
{
    void Method1();
    IDictionary<T, BaseClassItem<U>> GetDictionary();
}

I have one of the implementations like this

public class LruEvictor<T, U> : SomeInterface<T, U>
    where T : struct
    where U : class
{
    private Dictionary<T, BaseClassItem<U>> _dictionary;

    public void Evict()
    {

    }
    public IDictionary<T, BaseClassItem<U>> GetDictionary()
    {
        _dictionary = new Dictionary<T, BaseClassItem<U>>();
        return _dictionary;
    }

}

In the above GetDictionary() method I would like to return a dictionary of type Dictionary<T, DerivedItem<U>>.

Is that possible? If yes how do I do it.

Given below is the derived class implementation.

public class DerivedItem<U> : BaseClassItem<U>
    where U : class
{        

    public DerivedItem(U value) :base(value)
    {

    }
    public DateTime LastAccessed { get; private set; }

    public override void OnAccessed()
    {
        LastAccessed = DateTime.Now;
    }
}

Any input will be appreciated.

Thanks in advance.

Answer 1

I'm fairly sure you can't do this, as it would break the type system.

Consider the usual animal example:

public IDictionary<T, Animal> GetDictionary()
{
    _dictionary = new Dictionary<T, Llama>();
    return _dictionary; // Imagine you can do this
}

IDictionary<T, Animal> dict = GetDictionary();
dict.Add(MakeT(), new Horse()); // Ouch.

Answer 2

No, you can't do that. A simple version of your question would be that given

class Base { }
class Derived : Base { }

is it possible to use IDictionary<T, Base> dict = new Dictionary<T, Derived>() .

This is not possible because covariance rules of C# don't allow that. And for good reason because if it would work then you could simply add an object of type Base to the dictionary that should only accept Derived objects.