C#•2y ago

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•2y 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•2y 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•2y 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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Using a derived type in Action