1、在window窗体加个button控件，双击进去：

2、进入方法体中，编写方法

 private void btnDownload_Click(object sender, EventArgs e)
        {
            DialogResult rs = MessageBox.Show("是否确定下载文件？", "系统提示", MessageBoxButtons.YesNo,MessageBoxIcon.Information);
            if (rs == DialogResult.Yes)
            {
                MessageBox.Show("正在下载，请稍后。。。。。。");
                string URL = "http://localhost:8088/CS_Dsp.zip"; //这个是服务器资源
                string filename = @"D:\CS_Dsp.zip";    //这个下载到本地保存路径

　　　　　　//得到客户端请求的对象
                System.Net.HttpWebRequest Myrq = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(URL);

　　　　　　//得到浏览器响应的对象
                System.Net.HttpWebResponse myrp = (System.Net.HttpWebResponse)Myrq.GetResponse();
                long totalBytes = myrp.ContentLength;
                System.IO.Stream st = myrp.GetResponseStream();
                System.IO.Stream so = new System.IO.FileStream(filename, System.IO.FileMode.Create);
                long totalDownloadedByte = 0;
                byte[] by = new byte[1024];
                int osize = st.Read(by, 0, (int)by.Length);
                while (osize > 0)
                {
                    totalDownloadedByte = osize + totalDownloadedByte;
                    so.Write(by, 0, osize);
                    osize = st.Read(by, 0, (int)by.Length);
                }
                so.Close();
                st.Close();
                MessageBox.Show("下载文件成功！");
            }
            else
            {
                MessageBox.Show("取消下载！");
            }
        }

3、点击下载文件进行测试

注意：前提在WEB服务器url有你自己写的文件，才能下载下来。