C
C#•3y ago
Kiel

Using a derived type in Action

I'm deriving from an abstract class whose constructor accepts an object of type Action<LocalMessageBase>. LocalMessageBase is an abstract class, of which there are a few derived classes, of which one is LocalInteractionMessageResponse. Is there a way to use this derived type for the action instead of the base type, either directly within the action, or via some other casting magic? This is what I tried to no avail: 
public class MyView : ViewBase
{
  public MyView(Action<LocalInteractionMessageResponse> messageTemplate)
    : base(messageTemplate)
  { }
}
public class MyView : ViewBase
{
  public MyView(Action<LocalInteractionMessageResponse> messageTemplate)
    : base(messageTemplate)
  { }
}
though casting messageTemplate to Action<LocalMessageBase> doesn't cause any warning/hint lints in VS or via R#?
3 Replies
Kouhai
Kouhai•3y ago
You can cast Action<LocalInteractionMessageResponse>to Action<LocalMessageBase> at runtime it'll throw InvalidCastException You can however do something like this 
abstract class BaseAction<T> where T : LocalMessageBase
{
    public BaseAction(Action<T> action) { }
}

class LocalInteractionMessageResponseAction : BaseAction<LocalInteractionMessageResponse>
{
    public LocalInteractionMessageResponseAction(Action<LocalInteractionMessageResponse> action) : base(action) { }
}

class LocalInteractionMessageResponse : LocalMessageBase
{
}
abstract class LocalMessageBase
{
}
abstract class BaseAction<T> where T : LocalMessageBase
{
    public BaseAction(Action<T> action) { }
}

class LocalInteractionMessageResponseAction : BaseAction<LocalInteractionMessageResponse>
{
    public LocalInteractionMessageResponseAction(Action<LocalInteractionMessageResponse> action) : base(action) { }
}

class LocalInteractionMessageResponse : LocalMessageBase
{
}
abstract class LocalMessageBase
{
}
Kiel
KielOP•3y ago
how would this solution apply to my example exactly? if it removes any additional confusion, this ViewBase class I am deriving is third-party and I can't modify it, as well as any of the LocalMessageBase + derived classes
Kouhai
Kouhai•3y ago
If you can't modify them, I'd do 
class MyView : ViewBase
{
    public MyView(Action<LocalInteractionMessageResponse> action) : base((m) => action(m as LocalInteractionMessageResponse))
    {

    }
}
class MyView : ViewBase
{
    public MyView(Action<LocalInteractionMessageResponse> action) : base((m) => action(m as LocalInteractionMessageResponse))
    {

    }
}
Or if I'm going to be deriving from ViewBase a lot, I'd just do 
static class Extensions
{
    public static Action<LocalMessageBase> AsBase<T>(this Action<T> action) where T : LocalMessageBase
    {
        return (m) => action(m as T);
    }
}

class MyView : ViewBase
{
    public MyView(Action<LocalInteractionMessageResponse> action) : base(action.AsBase())
    {

    }
}
static class Extensions
{
    public static Action<LocalMessageBase> AsBase<T>(this Action<T> action) where T : LocalMessageBase
    {
        return (m) => action(m as T);
    }
}

class MyView : ViewBase
{
    public MyView(Action<LocalInteractionMessageResponse> action) : base(action.AsBase())
    {

    }
}
@Kiel

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