using System;
using System.Threading;
using System.Threading.Tasks;

namespace WindowsFormsApplication2
{
    public delegate void ChangedEventHandler(object sender, EventArgs e);

    public class WorkLongTime
    {
        public event ChangedEventHandler Changed;

        protected virtual void OnChanged(EventArgs e)
        {
            if (Changed != null)
                Changed(this, e);
        }

        public void WorkLong()
        {
            string[] myLittleArray = new string[10] {"abc","abc","abc","abc","abc","abc","abc","abc","abc","abc"};
            Parallel.ForEach(myLittleArray, singleString =>
                {
                    Thread.Sleep(500);
                    OnChanged(EventArgs.Empty);
                });
        }
    }
}