一、WebClient概述

   WebClient提供向 URI 标识的资源发送数据和从 URI 标识的资源接收数据的公共方法， WebClient 类提供向 URI 标识的任何本地、Intranet 或 Internet 资源发送数据以及从这些资源接收数据的公共方法。
   WebClient 类使用 WebRequest 类提供对资源的访问。WebClient 实例可以通过任何已向 WebRequest.RegisterPrefix 方法注册的 WebRequest 子代访问数据。默认情况下，WebClient 实例不发送可选的 HTTP 报头。
   如果您的请求需要可选报头，必须将该报头添加到 Headers 集合。例如，要在响应中保留查询，必须添加用户代理报头。此外，如果用户代理标头丢失，服务器可能返回 500（内部服务器错误）。
   在 WebClient 实例中，AllowAutoRedirect 设置为 true。
   

   查看MSND上对webclient的解释，有上传与下载方法：

WebClient提供一下上传方法：
   
WebClient提供一下下载方法：

二、WebClient与其他网络相关类的区别

WebClient和HttpWebRequst是用来获取数据的2种方式，一般而言，WebClient更倾向于“按需下载”，事实上掌握它也是相对容易的，而HttpWebRequst则允许你设置请求头或者对内容需要更多的控制，
后者有点类似于form中的submit。虽然两者都是异步请求事件，但是WebClient是基于事件的异步，而HttpWebRequst是基于代理的异步编程。

三、WebClient使用范例

文件下载：

 protected void Button1_Click(object sender, EventArgs e){String url = "http://localhost/SprayerWeb/download/1.zip";WebClient webclient = new WebClient();byte [] bytes = webclient.DownloadData(url);string path = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", DateTime.Now.ToString("yyyyHHmmHHmmssfff"), ".zip");FileStream fs = new FileStream(path, FileMode.Create);fs.Write(bytes, 0, bytes.Length);fs.Close();}protected void Button2_Click(object sender, EventArgs e){WebClient webclient = new WebClient();String url = "http://localhost/SprayerWeb/download/1.zip";Stream stream = webclient.OpenRead(url);string path = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", DateTime.Now.ToString("yyyyHHmmHHmmssfff"), ".zip");FileStream fs = new FileStream(path, FileMode.Create);byte[] bytes = new byte[8192];int read = 0;while ((read = stream.Read(bytes, 0, bytes.Length)) > 0) {fs.Write(bytes, 0, read);//这儿需要使用read,不能使用bytes.Length}fs.Close();}protected void Button3_Click(object sender, EventArgs e){WebClient webclient = new WebClient();String url = "http://localhost/SprayerWeb/download/1.zip";string path = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", DateTime.Now.ToString("yyyyHHmmHHmmssfff"), ".zip");webclient.DownloadFile(url, path);}protected void Button4_Click(object sender, EventArgs e){String url = "http://localhost/SprayerWeb/download/1.zip";Uri uri = new Uri(url, UriKind.Absolute);WebClient webclient = new WebClient();webclient.OpenReadAsync(uri);webclient.OpenReadCompleted+=webclient_OpenReadCompleted;}private void webclient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e){Stream stream = e.Result;byte[] bytes = new byte[8192];int read = 0;string path = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", DateTime.Now.ToString("yyyyHHmmHHmmssfff"), ".zip");FileStream fs = new FileStream(path, FileMode.Create);while ((read = stream.Read(bytes, 0, bytes.Length)) > 0) {fs.Write(bytes, 0, read);}fs.Close();stream.Close();}

文件上传：

 protected void Button5_Click(object sender, EventArgs e){//在IIS6下没有执行成功，以后再调试吧WebClient webclient = new WebClient();string targetpath = string.Format("{0}{1}{2}","http://localhost/WebApp/upload/",DateTime.Now.ToString("yyyyMMddHHmmssfff"),".zip");string sourcepath = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", "1", ".zip");byte[] responseArray = webclient.UploadFile(targetpath, "PUT", sourcepath);}protected void Button6_Click(object sender, EventArgs e){//在IIS6下没有执行成功，以后再调试吧string targetpath = string.Format("{0}{1}{2}", "http://localhost/WebApp/upload/", DateTime.Now.ToString("yyyyMMddHHmmssfff"), ".zip");string sourcepath = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", "1", ".zip");WebClient webclient = new WebClient();FileStream fs = new FileStream(sourcepath, FileMode.Open, FileAccess.Read);byte[] bt = new byte[fs.Length];fs.Read(bt, 0, bt.Length);byte[] responseArray = webclient.UploadData(targetpath, "PUT", bt);fs.Close();}protected void Button7_Click(object sender, EventArgs e){WebClient webclient = new WebClient();string sourcepath = string.Format("{0}\\{1}\\{2}{3}", Server.MapPath("~"), "download", "1", ".zip");byte[] responseArray= webclient.UploadFile("http://localhost/WebApp/upload.aspx", "POST", sourcepath);Response.Write("result:"+System.Text.Encoding.UTF8.GetString(responseArray));}

upload.aspx文件代码如下：

 protected void Page_Load(object sender, EventArgs e){//string uploadpath = string.Format("{0}\\{1}{}");foreach (string f in Request.Files.AllKeys){HttpPostedFile postfile = Request.Files[f];string path = String.Format("{0}\\{1}\\{2}{3}",Server.MapPath("~"),"upload",DateTime.Now.Ticks.ToString(),".zip");postfile.SaveAs(path);}Response.Write("Success");}

更多详细内容参考:https://msdn.microsoft.com/zh-cn/library/system.net.webclient%28VS.80%29.aspx