Not able to attach multiple files while sending email

private void button4_Click(object sender, RoutedEventArgs e)
      {
          Microsoft.Win32.OpenFileDialog filedialog = new
          Microsoft.Win32.OpenFileDialog();
          filedialog.Filter = "Zip Files|*.zip";
          if (flag == 0)
          {
              bool b = (bool)filedialog.ShowDialog();
              if (b == true)
              {
                  attach.Text = filedialog.FileName+";";
              }
              flag++;
          }
          else
          {
              TextBox vv = new TextBox();
              bool b = (bool)filedialog.ShowDialog();
              if (b == true)
              {
                  attach.Text += filedialog.FileName + ";";
              }
          }


      }

foreach (string m in attach.text.Split(';'))
                 {
                     
                     mail.Attachments.Add(new Attachment(m));

                 }