Not able to access event defined in a class-CodePudding

I am trying to subscribe a event but it is showing error on the mentioned line. Can anyone tell me what the problem in my code is?

I am new to programming, so kindly please provide a detailed answer. Thanks in advance

namespace Events
{
    delegate void MyEventHandler();

    internal class ArrayListExamplePublisher : ArrayList
    {
        public event MyEventHandler objMyEventHandler;

        void OnAdded()
        {
            if (objMyEventHandler != null)
            {
                objMyEventHandler();
            }
        }

        public override int Add(object? value)
        {
            OnAdded();
            return base.Add(value);
        }
    }

    public class ArrayListExample
    {
        static void Main()
        {
            ArrayList arrayList = new ArrayList();

            // Here I get an error
            arrayList.objMyEventHandler  = () => Console.WriteLine("Object Added"); 
            
            arrayList.Add(1);
            arrayList.Add("4");
        }
    }
}

CodePudding user response：

You are adding the event to ArrayListExamplePublisher, not ArrayList, so when you try to assign that event on an ArrayList, it fails.

You can't add an event to a class externally in C# - you'd have to modify the definition of ArrayList, which you can't do.

You could do:

ArrayListExamplePublisher arrayList = new ArrayListExamplePublisher();

and set the event on that, but it's not clear if that's what you want.

Side note: The framework already has ObservableCollection<T> that has events for when the collection is modified (and is strongly-typed, unlike ArrayList.

CodePudding user response：

Your lambda expression has to match the delegate type for the event.

Most events take two arguments, so this should probably work:

arrayList.objMyEventHandler  = ( s, e ) => Console.WriteLine("Object Added");

CodePudding user response：

So i have figured it out As i am inheriting the class ArrayList so I should use object of child class but i was using the object of parent class. This works perfectly

ArrayListExamplePublisher objArrayListExamplePublisher = new ArrayListExamplePublisher();
            objArrayListExamplePublisher.Added  = () => Console.WriteLine("Object Added");
            objArrayListExamplePublisher.Add(1);
            objArrayListExamplePublisher.Add("4");