using System;
using System.Drawing;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            Button b = new Button();
            b.Click += b_Click;
            this.Controls.AddRange(new Control[] { b, new TextBox() { Name = "EventText", Location = new Point(0, 30) } });
        }

        void b_Click(object sender, EventArgs e)
        {
            Class1 c1 = new Class1();
            c1.Success += c1_Success;
            c1.Start();
        }

        private void c1_Success(object sender, MyEvent e)
        {
            TextBox t = this.Controls["EventText"] as TextBox;
            if (t != null)
                this.Invoke(new Action(() => { t.Text = e.RetData; }));
        }
    }

    public class Class1
    {
        public EventHandler<MyEvent> Success;

        public void Start()
        {
            Task t = Task.Factory.StartNew(() => foo());
        }

        private void foo()
        {
            OnSuccess(new MyEvent());
        }

        public void OnSuccess(MyEvent e)
        {
            var ret = Success;
            if (ret != null)
            {
                ret(this, e);
            }
        }
    }

    public class MyEvent : EventArgs
    {
        public string RetData = "Success!";
    }
}

private void foo()
{
     OnSuccess(new MyEvent());
}

this.Invoke(new Action(() => { t.Text = e.RetData; }));

t.Text = e.RetData;