Abstract collection of base class

Question

I'm creating an WPF application using the MVVM framework.

I've got several classes all inheriting from a base class "MessageParentBase"

public class ChatMessage : MessageParentBase
{
}

public class CargoMessage : MessageParentBase
{
}

The ViewModels all inherit once again from a base class "ParentMessageWindowBaseViewModel"

public class UserControlChatViewModel : ParentMessageWindowBaseViewModel
{
}

public class UserControlCargoViewModel : ParentMessageWindowBaseViewModel
{
}

Now what I want to do is have an observable collection in my "ParentMessageWindowBaseViewModel" class

abstract internal ObservableCollection<MessageParentBase> GridData
{
    get; set;
}

Which is there so I can have a generic method at the "ParentMessageWindowBaseViewModel" level, but at the child classes I want to have a more specific collection declared

internal override ObservableCollection<ChatMessage > GridData
{
    get { return _GridData; }
    set
    {
        _GridData = value;
    }
}

Visual studio is telling me it must be declared as a type of "MessageParentBase".

Can someone please explain to me how I can achieve this as I'm struggling at the moment.

Many thanks for your help.

Answer 1

A ObservableCollection< ChatMessage> isn't the same as ObservableCollection< MessageParentBase> so you can't override it.

You could use this like this:

public class MessageParentBase
{
}

public class ChatMessage : MessageParentBase
{
}

public abstract class ParentMessageWindowBaseViewModel<T> where T : MessageParentBase
{
    abstract internal ObservableCollection<T> GridData
    {
        get;
        set;
    }
}

public class Child : ParentMessageWindowBaseViewModel<ChatMessage>
{

    internal override ObservableCollection<ChatMessage> GridData
    {
        get;
        set;
    }
}

But storing all Childs in 1 list isn't possible. So it won't fix the issue with inheritance on a generic property.

Answer 2

In the class where you declare the property GridData to be abstract , why do you also introduce a backing field _GridData there? Don't do that.

If you want the property to be abstract , do not put any supporting field there. If you want to implement the property at the "high" level (base class), remove the abstract modifier from it.