CustomEvent.cs

using System;

public class CustomEvent
{
   object eventLock = new Object();
   
   public event EventHandler CustomEvt
   {
      add
      {
         lock (eventLock)
            {
               Console.WriteLine("Add called");
               CustomEvt += value;
            }
      }
      remove
      {
         lock (eventLock)
            {
               Console.WriteLine("Remove vcalled");
               CustomEvt -= value;
            }
      }
   }

   public event EventHandler PlainEvt;

   public void OnCustomEvt(EventArgs e)
   {
       // Breaks with error CS0079: The event "CustomEvent.ChangedEvt" can only appear on the left hand side of "+=" or "-=" operator
       CustomEvt(this, e);
   }

   public void OnPlainEvt(EventArgs e)
   {
       PlainEvt(this, e);
   }

    public static void Main()
    {
    }
}