Question

In C# I have a bunch of objects all inheriting from the same base class.
I also have a number of Dictionaries, one for each subclass.
What I want to do is add all of those Dictionaries to one List so I can loop through them all and do some work (like comparing the lists etc).

In summary

Dictionary<string, Child> childObjects = new Dictionary<string, Child>();
List<Dictionary<string, Parent>> listOfDictionaries = new List<Dictionary<string, Parent>>();
listOfDictionaries.Add(childObjects);

I would have thought that since Child inherits from Parent, this should work, but it won’t compile. Clearly I am not understanding something about inheritance and generics 🙂

A full code example

class Program
{
    static void Main(string[] args)
    {
        //Creating a Dictionary with a child object in it
        Dictionary<string, Child> childObjects = new Dictionary<string, Child>();
        var child = new Child();
        childObjects.Add(child.id, child);

        //Creating a "parent" Dictionary with a parent and a child object in it
        Dictionary<string, Parent> parentObjects = new Dictionary<string, Parent>();
        parentObjects.Add(child.id, child);
        var parent = new Parent();
        parentObjects.Add(parent.id, parent);

        //Adding both dictionaries to a general list
        List<Dictionary<string, Parent>> listOfDictionaries = new List<Dictionary<string, Parent>>();

        listOfDictionaries.Add(childObjects);  //This line won't compile

        listOfDictionaries.Add(parentObjects);


    }
}

class Parent
{
    public string id { get; set; }
    public Parent()
    {
        this.id = "1";
    }
}

class Child : Parent
{
    public Child()
    {
        this.id = "2";
    }

}

Is there any way of achieving this?

Answer 1

You can’t do this safely. Imagine you did this:

listOfDictionaries[0]["foo"] = new Parent();

That looks fine – but it would mean that childObjects would contain a value which isn’t an instance of Child!

C# 4 has introduced restricted generic variance where it’s safe – so you can convert a reference of type IEnumerable<Banana> to IEnumerable<Fruit> for example – but what you’re wanting to do here isn’t safe, so it still isn’t allowed.

If you could tell us more about the bigger context – what you’re trying to achieve – we may be able to help more. Can you give examples of what you’d want to do with the list afterwards?