CaptchaRecogition.gp_down.btn_down_Click C# (CSharp) Method

        private void btn_down_Click(object sender, EventArgs e)
        {
            if (!string.IsNullOrEmpty (textBox1.Text .Trim ()))
            {
                int num = int.Parse(textBox1.Text.Trim());
                if (!string .IsNullOrEmpty (downloadPath ))
                {
                    Random r = new Random();
                    string url;
                    for (int i = 0; i <num ; i++)
                    {
                     url = txt_Url.Text + r.Next(1, 9999);
                        SaveCodeImageFromUrl(url, downloadPath+"\\"+i+".jpg");
                    }
                    MessageBox.Show("下载完毕！");

                }
                else
                {
                    MessageBox.Show("没有选择保存路径");
                }
            }
            else
            {
                MessageBox.Show("数量没有填！");
            }
        }