C#异步执行带有返回值和参数的方法，且获取返回值

 //获取所有的邮件
        private List GetEmailOnlyCount(POP3_ClientMessageCollection pOP3Collection)
        {
            int i = 0;
            List list = new List();
            foreach (POP3_ClientMessage message in pOP3Collection)
            {
                byte[] bytes = System.Text.Encoding.Default.GetBytes(message.HeaderToString());
                Mail_Message mail_Message = Mail_Message.ParseFromByte(bytes);
                EmailModel emailModel = new EmailModel()
                {
                    EmailFrom = mail_Message.From.ToString(),
                    EmailDate = mail_Message.Date.ToString(),
                    EmailSubject = mail_Message.Subject.ToString(),
                };
                list.Add(emailModel);
　　　　//异步线程中，改变主线程中的控件属于跨线程的调用，需要利用代理方法
                textBox5.Invoke(showProcessDelegate, "已经读取" + (++i) + "/" + pOP3Collection.Count + "封邮件");
            }
            return list;
        }

 private delegate List GetMailDelegate(POP3_ClientMessageCollection pOP3_ClientMessage);
 private GetMailDelegate getMailDelegate = new GetMailDelegate(GetEmailOnlyCount);

private AsyncCallback asyncCallback;//执行完的回调方法，异步方法执行完后执行的方法

getMailDelegate.BeginInvoke(pOP3_ClientMessageCollection, asyncCallback, null);

asyncCallback = delegate (IAsyncResult result)
            {
                EmaliList = getMailDelegate.EndInvoke(result);//关闭异步，并且获取到方法的返回值
                for (int i = EmaliList.Count; i > 0; i--)
                {
                    ListViewItem listViewItem = new ListViewItem();
                    listViewItem.Text = EmaliList[i - 1].EmailFrom;
                    listViewItem.SubItems.Add(EmaliList[i - 1].EmailSubject);
                    listViewItem.SubItems.Add(EmaliList[i - 1].EmailDate);
                    listViewItem.SubItems.Add(EmaliList[i - 1].AttchmentsCount.ToString());
                    listView1.Invoke(showListViewDelegate, listViewItem);
                }

            };