How do I define an EventHandler in an abstract class using an event inherited from a base class?

Question

My intent is to reuse the SelectedValueChanged event inherited from the ComboBox Class (which, in turn, inherits it from the ListControl Class)

In the code below: SelectedValueChanged is tagged with the compiler error shown in the screen shot. I do not intend on hiding the inherited event, so I do not want to use the new keyword. I want the classes that I derive from DRT_ComboBox_Abstract to be able to use the inherited event as-is.

How do I define an EventHandler using an event inherited from a base class? (Or, am I totally off the planet with regard to understanding events?)

Note: "Show Potential Fixes" surrounds public event EventHandler SelectedValueChanged with #pragma warning disable CS0108 which just disables the warning.

screen shot

using System;
using System.Windows.Forms;

namespace DRT
{
    internal abstract partial class DRT_ComboBox_Abstract : ComboBox
    {
        //SelectedValueChanged is tagged with the compiler error shown in the screenshot
        public event EventHandler SelectedValueChanged;

        public DRT_ComboBox_Abstract()
        {
            InitializeComponent();
        }

        public void Disable()
        {
            this.Enabled = false;
        }

        public void _OnSelectedValueChanged(object sender, System.EventArgs e)
        {
            this.SelectedValueChanged?.Invoke(sender, e);
        }
    }
}

But you are not doing anything with the event so why even declare it in the inherited class? — CodingYoshi, May 21 '18 at 19:26
@CodingYoshi The main form class subscribes to the event. I did not include main form class code in my question — VA systems engineer, May 22 '18 at 11:28

Diego Rafael Souza · Accepted Answer · 2018-05-21T19:54:59.717

You don't need to declare the event again. If it is public and it is being already thrown when it needs, you can handle the changes if you need, by subscribing to the base class event.

I mean, you can do something like:

using System;
using System.Windows.Forms;

namespace DRT
{
    internal abstract partial class DRT_ComboBox_Abstract : ComboBox
    {
        public DRT_ComboBox_Abstract()
        {
            InitializeComponent();
            SelectedValueChanged += MyOwnHandler
        }

        protected virtual void MyOwnHandler(object sender, EventArgs args)
        {
            // Hmn.. now I know that the selection has changed and can so somethig from here
            // This method is acting like a simple client
        }
    }
}

On SOLID classes (I believe it is the case for ComboBox), often the method that effectively invokes the subscribers to handle some event is usually virtual, allowing you, once you're inheriting from this class, intercept the event handlers invocation, if it is what you want.

It is:

using System;
using System.Windows.Forms;

namespace DRT
{
    internal abstract partial class DRT_ComboBox_Abstract : ComboBox
    {
        public DRT_ComboBox_Abstract()
        {
            InitializeComponent();
        }

        protected override void OnSelectedValueChanged(object sender, EventArgs args)
        {
            // Wait, the base class is attempting to notify the subscribers that Selected Value has Changed! Let me do something before that
            // This method is intercepting the event notification

            // Do stuff

            // Continue throwing the notification
            base.OnSelectedValueChanged(sender, args);
        }
    }
}

^{Note: I've removed the Disable method just for code simplification. It's out of scope in this subject}

I hope it helps.

It does. I'm an old procedural programmer. It taking time to get my head into an OO frame of mind. `SelectedValueChanged += MyOwnHandler` should have been obvious :-) Also, thanks for the pointer to SOLID — VA systems engineer, May 22 '18 at 11:26

How do I define an EventHandler in an abstract class using an event inherited from a base class?

1 Answers1