我试图从URL下载文件,我必须在WebClient和HttpClient之间进行选择。我在互联网上引用了this文章和其他几篇文章。无处不在,由于其非常好的异步支持和其他.Net 4.5权限,建议使用HttpClient。但我仍然不完全相信并需要更多的投入。
我使用下面的代码从互联网上下载文件:
Web客户端:
WebClient client = new WebClient();
client.DownloadFile(downloadUrl, filePath);
HttpClient的:
using (HttpClient client = new HttpClient())
{
using (HttpResponseMessage response = await client.GetAsync(url))
using (Stream streamToReadFrom = await response.Content.ReadAsStreamAsync())
{
}
}
从我的角度来看,我只能看到使用WebClient的一个缺点,那就是非异步调用,阻塞调用线程。但是,如果我不担心阻塞线程或使用client.DownloadFileAsync()
来利用异步支持,该怎么办?
另一方面,如果我使用HttpClient,是不是我将文件的每个字节加载到内存中然后将其写入本地文件?如果文件太大,内存开销不会很贵吗?如果我们使用WebClient可以避免这种情况,因为它将直接写入本地文件而不会消耗系统内存。
那么,如果性能是我的首要任务,我应该使用哪种方法进行下载?如果我的上述假设是错误的,我想澄清一下,我也愿意接受替代方法。
这是我的方法。
如果您正在调用Web Api来获取文件,那么从控制器方法可以使用HttpClient GET请求并使用FileStreamResult返回类型返回文件流。
public async Task<ActionResult> GetAttachment(int FileID)
{
UriBuilder uriBuilder = new UriBuilder();
uriBuilder.Scheme = "https";
uriBuilder.Host = "api.example.com";
var Path = "/files/download";
uriBuilder.Path = Path;
using (HttpClient client = new HttpClient())
{
client.BaseAddress = new Uri(uriBuilder.ToString());
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Add("authorization", access_token); //if any
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
HttpResponseMessage response = await client.GetAsync(uriBuilder.ToString());
if (response.IsSuccessStatusCode)
{
System.Net.Http.HttpContent content = response.Content;
var contentStream = await content.ReadAsStreamAsync(); // get the actual content stream
return File(contentStream, content_type, filename);
}
else
{
throw new FileNotFoundException();
}
}
}
Here’s one way to use it to download a URL and save it to a file :(我使用的是Windows 7,因此我没有WindowsRT,所以我也使用System.IO。)
public static class WebUtils
{
private static Lazy<IWebProxy> proxy = new Lazy<IWebProxy>(() => string.IsNullOrEmpty(Settings.Default.WebProxyAddress) ? null : new WebProxy { Address = new Uri(Settings.Default.WebProxyAddress), UseDefaultCredentials = true });
public static IWebProxy Proxy
{
get { return WebUtils.proxy.Value; }
}
public static Task DownloadAsync(string requestUri, string filename)
{
if (requestUri == null)
throw new ArgumentNullException(“requestUri”);
return DownloadAsync(new Uri(requestUri), filename);
}
public static async Task DownloadAsync(Uri requestUri, string filename)
{
if (filename == null)
throw new ArgumentNullException("filename");
if (Proxy != null)
WebRequest.DefaultWebProxy = Proxy;
using (var httpClient = new HttpClient())
{
using (var request = new HttpRequestMessage(HttpMethod.Get, requestUri))
{
using (Stream contentStream = await (await httpClient.SendAsync(request)).Content.ReadAsStreamAsync(), stream = new FileStream(filename, FileMode.Create, FileAccess.Write, FileShare.None, Constants.LargeBufferSize, true))
{
await contentStream.CopyToAsync(stream);
}
}
}
}
}
请注意,代码正在保存我在设置中使用的代理服务器的地址(在工作中),如果指定了此类设置,则使用该代码。否则,它应告诉您有关使用HttpClient beta下载和保存文件的所有信息。
对于重复调用的代码,您不希望将HttpClient
放在using
块中(it will leave hanging ports open)
为了使用HttpClient下载文件,我发现this extension method对我来说似乎是一个好的和可靠的解决方案:
public static class HttpContentExtensions
{
public static Task ReadAsFileAsync(this HttpContent content, string filename, bool overwrite)
{
string pathname = Path.GetFullPath(filename);
if (!overwrite && File.Exists(filename))
{
throw new InvalidOperationException(string.Format("File {0} already exists.", pathname));
}
FileStream fileStream = null;
try
{
fileStream = new FileStream(pathname, FileMode.Create, FileAccess.Write, FileShare.None);
return content.CopyToAsync(fileStream).ContinueWith(
(copyTask) =>
{
fileStream.Close();
});
}
catch
{
if (fileStream != null)
{
fileStream.Close();
}
throw;
}
}
}
您可以使用.Net 4.5+本地执行此操作。我尝试按照你的方式进行,然后我在Intellisense中找到了一个似乎有意义的方法。
https://docs.microsoft.com/en-us/dotnet/api/system.io.stream.copytoasync?view=netframework-4.7.2
uri = new Uri(generatePdfsRetrieveUrl + pdfGuid + ".pdf");
HttpClient client = new HttpClient();
var response = await client.GetAsync(uri);
using (var fs = new FileStream(
HostingEnvironment.MapPath(string.Format("~/Downloads/{0}.pdf", pdfGuid)),
FileMode.CreateNew))
{
await response.Content.CopyToAsync(fs);
}