//1.   怎样获取一个字符串中空格之前的数据.
//      str为要截取的字符串
if(str.IndexOf(' ')>-1)
    Response.Write(str.Substring(0,str.IndexOf(' ')));

//2. 怎样通过按钮的点击事件,自动发送邮件到相应的邮箱.  
//在button的click事件内
System.Net.Mail.MailMessage mailMessage = new System.Net.Mail.MailMessage();
            mailMessage.To.Add("xxx@xxx.com"); //发到哪个邮箱?
            mailMessage.From = new MailAddress("xxx@xxx.com", "xxx", System.Text.Encoding.UTF8);        // 这3个参数分别是发件人地址（可以随便写），发件人姓名，编码*/
            mailMessage.Subject = "xxx";//邮件标题 
            mailMessage.SubjectEncoding = System.Text.Encoding.UTF8;//邮件标题编码 
            
            mailMessage.BodyEncoding = System.Text.Encoding.UTF8;//邮件内容编码 
            mailMessage.IsBodyHtml = false;//是否是HTML邮件 
            mailMessage.Priority = MailPriority.High;//邮件优先级 
            mailMessage.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;
            //附件
            Attachment attachment = new Attachment(@"xxx", System.Net.Mime.MediaTypeNames.Application.Octet); //附件
          mailMessage.Attachments.Add(attachment);
          mailMessage.Body = "xxx";//邮件内容 
            SmtpClient smtpClient = new SmtpClient();
          smtpClient.Credentials = new System.Net.NetworkCredential("xxx@xxx.com", "xxx");//发邮件的邮箱和密码 
            smtpClient.Host = "smtp.163.com"; //发邮件的邮箱的smtp,偶这里用的163邮箱
           smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
          smtpClient.Send(mailMessage);

------解决方案--------------------
还是楼上详尽

------解决方案--------------------
string.Substring( 0, string.IndexOf( ' ' ) )

------解决方案--------------------
ddl.Items.Clear()