C#

....
public class MultiContainer : MarshalByRefObject, IMultiContainer
    {
        public string GetClientsList()
        {
            Thread.Sleep(1000);
            return Program.ServerForm.GetClientsList();
        }
....

namespace Client
{
    public partial class Form1 : Form
    {
        Server.IMultiContainer mc;
        private delegate string Delegate();

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            mc = new Server.MultiContainer();
            Delegate del = new Delegate(mc.GetClientsList);
            AsyncCallback callback = new AsyncCallback(Callback);
            del.BeginInvoke(callback, null);
        }

        private void Callback(IAsyncResult ar)
        {
            //this callback method is never called...
            Delegate del = (Delegate)((AsyncResult)ar).AsyncDelegate;
            richTextBox1.Text = del.EndInvoke(ar);
        }
...