异步调用方法

        //声明委托
        public delegate void AsyncEventHandler (object sender);

        [STAThread]
        public void DownLoadToExcel (object sender)
        {
            //实例委托
            AsyncEventHandler asy = new AsyncEventHandler (downLoad);
            //异步调用开始，没有回调函数和AsyncState,都为null
            IAsyncResult ia = asy.BeginInvoke (sender,new AsyncCallback (CallbackMethod),asy);
        }

        //回调方法
        public void CallbackMethod (IAsyncResult ar)
        {
            AsyncEventHandler dlgt = (AsyncEventHandler)ar.AsyncState;
            dlgt.EndInvoke (ar);
            MessageBox.Show ("下载完成");
        }

       //异步执行的方法

       public void downLoad (object sender)
        {
            DataTable dt = selectAllPayLog (this.dateTimestart.Value,this.dateTimeEnd.Value);
            if (dt != null && dt.Rows.Count != 0)
                DataBaseToExcel.DataToExcel (dt,@"E:\DataToExcel" + DateTime.Now.ToString ("yyyyMMdd") + ".xls");
        }