using System.Net;

WebClient webClient = new WebClient();
webClient.DownloadFile("http://mysite.com/myfile.txt", @"c:\myfile.txt");

在.NET Core MVC中，你有时可以简单地这样做:

public async Task<ActionResult> DownloadUrl(string url) {
    return Redirect(url);
}

这可能假设您正在尝试下载的MIME类型被浏览器设置为可下载(例如.mp4)，因此它不会尝试重定向到网页。

WebClient已经过时了

如果你想下载到一个文件，通过使用ResponseHeadersRead避免第一次读取到内存，就像这样:

static public async Task HttpDownloadFileAsync(HttpClient httpClient, string url, string fileToWriteTo) {
  using HttpResponseMessage response = await httpClient.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);
  using Stream streamToReadFrom = await response.Content.ReadAsStreamAsync(); 
  using Stream streamToWriteTo = File.Open(fileToWriteTo, FileMode.Create); 
  await streamToReadFrom.CopyToAsync(streamToWriteTo);
}

以上代码更多的是一个大纲，添加正确的错误/异常处理是不平凡的，进度报告也是不平凡的，因为是处置。

我提出了一组c# 9.0扩展类downadfileasync, GetToStringAsync和PostToStringAsync

namespace System.Net.Http {

  // HttpResponse is in one of 3 states:
  // - ResponseMessageInfo is object && ResponseMessageInfo.IsSuccessStatusCode -> success, inspect ResponseMessageInfo for StatusCode etc
  // - ResponseMessageInfo is object && !ResponseMessageInfo.IsSuccessStatusCode -> failure, inspect ResponseMessageInfo for StatusCode, ReasonPhrase etc
  // - ResponseMessageInfo is null -> exception, inspect ExceptionInfo fields
  public record HttpResponse {

    // copies of HttpRequestMessage and HttpResponseMessage which do not have the content and do not need to be disposed
    public record HttpRequestMessageInfo(HttpRequestHeaders Headers, HttpMethod Method, HttpRequestOptions Options, Uri? RequestUri, Version Version, HttpVersionPolicy VersionPolicy);
    public record HttpResponseMessageInfo(HttpResponseHeaders Headers, bool IsSuccessStatusCode, string? ReasonPhrase, HttpRequestMessageInfo RequestMessage, HttpStatusCode StatusCode, HttpResponseHeaders TrailingHeaders, Version Version);

    // holds Http exception information
    public record HttpExceptionInfo(HttpRequestMessageInfo HttpRequestMessage, string ErrorMessage, WebExceptionStatus? WebExceptionStatus);

    // if ResponseMessageInfo is null ExceptionInfo is not and vice versa
    public HttpResponseMessageInfo? ResponseMessageInfo { get; init; }
    public HttpExceptionInfo? ExceptionInfo { get; init; }

    public HttpResponse(HttpRequestMessage requestMessage, HttpResponseMessage responseMessage) {
      var requestMessageInfo = new HttpRequestMessageInfo(requestMessage.Headers, requestMessage.Method, requestMessage.Options, requestMessage.RequestUri, requestMessage.Version, requestMessage.VersionPolicy);
      ResponseMessageInfo = new(responseMessage.Headers, responseMessage.IsSuccessStatusCode, responseMessage.ReasonPhrase, requestMessageInfo, responseMessage.StatusCode, responseMessage.TrailingHeaders, responseMessage.Version);
      ExceptionInfo = null;
    }

    public HttpResponse(HttpRequestMessage requestMessage, Exception exception) {
      ResponseMessageInfo = null;
      var requestMessageInfo = new HttpRequestMessageInfo(requestMessage.Headers, requestMessage.Method, requestMessage.Options, requestMessage.RequestUri, requestMessage.Version, requestMessage.VersionPolicy);

      if (exception is WebException ex1 && ex1.Status == WebExceptionStatus.ProtocolError) {
        using HttpWebResponse? httpResponse = (HttpWebResponse?)ex1.Response;
        ExceptionInfo = new(requestMessageInfo, httpResponse?.StatusDescription ?? "", ex1.Status);
      } 
      else if (exception is WebException ex2) ExceptionInfo = new(requestMessageInfo, ex2.FullMessage(), ex2.Status);
      else if (exception is TaskCanceledException ex3 && ex3.InnerException is TimeoutException) ExceptionInfo = new(requestMessageInfo, ex3.InnerException.FullMessage(), WebExceptionStatus.Timeout);
      else if (exception is TaskCanceledException ex4) ExceptionInfo = new(requestMessageInfo, ex4.FullMessage(), WebExceptionStatus.RequestCanceled);
      else ExceptionInfo = new(requestMessageInfo, exception.FullMessage(), null);
    }

    public override string ToString() {
      if (ResponseMessageInfo is object) {
        var msg = ResponseMessageInfo.IsSuccessStatusCode ? "Success" : "Failure";
        msg += $" {Enum.GetName(typeof(HttpStatusCode), ResponseMessageInfo.StatusCode)}";
        if (ResponseMessageInfo.ReasonPhrase is object) msg += $" {ResponseMessageInfo.ReasonPhrase}";
        return msg;

      } else if (ExceptionInfo is object) {
        var msg = "Failure";
        msg += $" {ExceptionInfo.ErrorMessage}";
        if (ExceptionInfo.WebExceptionStatus is object) msg += $" {Enum.GetName(typeof(WebExceptionStatus), ExceptionInfo.WebExceptionStatus)}";
        return msg;
      }
      return "NA"; // never reach here
    }
  }


  public static class ExtensionMethods {

    // progressCallback recieves (bytesRecieved, percent, speedKbSec) and can return false to cancell download
    public static async Task<(bool success, HttpResponse httpResponse)> DownloadFileAsync(this HttpClient httpClient, Uri requestUri, string fileToWriteTo, CancellationTokenSource? cts = null, Func<long, int, float, bool>? progressCallback = null) {
      var httpRequestMessage = new HttpRequestMessage { Method = HttpMethod.Get, RequestUri = requestUri };
      var created = false;

      try {
        var cancellationToken = cts?.Token ?? default;

        using HttpResponseMessage httpResponseMessage = await httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
        if (!httpResponseMessage.IsSuccessStatusCode) return (false, new(httpRequestMessage, httpResponseMessage));
        var contentLength = httpResponseMessage.Content.Headers.ContentLength;

        using Stream streamToReadFrom = await httpResponseMessage.Content.ReadAsStreamAsync();
        using Stream streamToWriteTo = File.Open(fileToWriteTo, FileMode.Create);
        created = true;

        var buffer = new byte[81920]; 
        var bytesRecieved = (long)0;
        var stopwatch = Stopwatch.StartNew();
        int bytesInBuffer;
        while ((bytesInBuffer = await streamToReadFrom.ReadAsync(buffer, cancellationToken)) != 0) {
          await streamToWriteTo.WriteAsync(buffer.AsMemory(0, bytesInBuffer), cancellationToken);
          bytesRecieved += bytesInBuffer;
          if (progressCallback is object) {
            var percent = contentLength is object && contentLength != 0 ? (int)Math.Floor(bytesRecieved / (float)contentLength * 100.0) : 0;
            var speedKbSec = (float)((bytesRecieved / 1024.0) / (stopwatch.ElapsedMilliseconds / 1000.0));
            var proceed = progressCallback(bytesRecieved, percent, speedKbSec);
            if (!proceed) {
              httpResponseMessage.ReasonPhrase = "Callback cancelled download";
              httpResponseMessage.StatusCode = HttpStatusCode.PartialContent;
              return (false, new(httpRequestMessage, httpResponseMessage));
            }
          }
        }

        return (true, new(httpRequestMessage, httpResponseMessage));
      }
      catch (Exception ex) {
        if (created) try { File.Delete(fileToWriteTo); } catch { };
        return (false, new(httpRequestMessage, ex));
      }
    }

    public static async Task<(string? ResponseAsString, HttpResponse httpResponse)> GetToStringAsync(this HttpClient httpClient, Uri requestUri, CancellationTokenSource? cts = null) {
      var httpRequestMessage = new HttpRequestMessage { Method = HttpMethod.Get, RequestUri = requestUri };
      try {
        var cancellationToken = cts?.Token ?? default;
        using var httpResponseMessage = await httpClient.SendAsync(httpRequestMessage, cancellationToken);
        if (!httpResponseMessage.IsSuccessStatusCode) return (null, new(httpRequestMessage, httpResponseMessage));
        
        var responseAsString = await httpResponseMessage.Content.ReadAsStringAsync();
        return (responseAsString, new(httpRequestMessage, httpResponseMessage));
      }
      catch (Exception ex) {
        return (null, new(httpRequestMessage, ex)); ;
      }
    }

    public static async Task<(string? ResponseAsString, HttpResponse httpResponse)> PostToStringAsync(this HttpClient httpClient, Uri requestUri, HttpContent postBuffer, CancellationTokenSource? cts = null) {
      var httpRequestMessage = new HttpRequestMessage { Method = HttpMethod.Post, RequestUri = requestUri, Content = postBuffer };
      try {
        var cancellationToken = cts?.Token ?? default;
        using var httpResponseMessage = await httpClient.SendAsync(httpRequestMessage, cancellationToken);
        if (!httpResponseMessage.IsSuccessStatusCode) return (null, new(httpRequestMessage, httpResponseMessage));

        var responseAsString = await httpResponseMessage.Content.ReadAsStringAsync();
        return (responseAsString, new(httpRequestMessage, httpResponseMessage));
      }
      catch (Exception ex) {
        return (null, new(httpRequestMessage, ex));
      }
    }

  }
}

namespace System {
  public static class ExtensionMethods {
    public static string FullMessage(this Exception ex) {
      if (ex is AggregateException aex) return aex.InnerExceptions.Aggregate("[ ", (total, next) => $"{total}[{next.FullMessage()}] ") + "]";
      var msg = ex.Message.Replace(", see inner exception.", "").Trim();
      var innerMsg = ex.InnerException?.FullMessage();
      if (innerMsg is object && innerMsg!=msg) msg = $"{msg} [ {innerMsg} ]";
      return msg;
    }
  }
}

使用方法:

// download to file
var lastPercent = 0;
bool progressCallback(long bytesRecieved, int percent, float speedKbSec) {
  if (percent > lastPercent) {
    lastPercent = percent;
    Log($"Downloading... {percent}% {speedKbSec/1024.0:0.00}Mbps");
  }
  return true;
}

var (success, httpResponse) = await httpClient.DownloadFileAsync(
  new(myUrlString), 
  localFileName, 
  null, // CancellationTokenSource 
  progressCallback
);

if (success) {
  // file downloaded to localFile, httpResponse.ResponseMessageInfo contain 
  // extra information ie headers and status code

} else {
  Log(httpResponse.ToString()); // human friendly error information
  // if httpResponse.ResponseMessageInfo is object then server refused the request - 
  // examine httpResponse.ResponseMessageInfo.HttpStatusCode etc
  // else we had a Http exception - examine httpResponse.ExceptionInfo 
}


// Http get
var (responseAsString, httpResponse) = await httpClient.GetToStringAsync(url);
if (responseAsString is object) {
  // responseAsString contains the string response from the server

} else {
  // as for DownloadFileAsync
}


// http post
var postBuffer = new StringContent(jsonInString, System.Text.Encoding.UTF8, "application/x-www-form-urlencoded");
var (responseAsString, httpResponse) = await httpClient.PostToStringAsync(url, postBuffer);

if (responseAsString is object) {
  // responseAsString contains the string response from the server

} else {
  Log(httpResponse.ToString()); // human friendly error informaiton
  // as for DownloadFileAsync
}

如果你需要设置头文件和cookie来下载文件，你需要做一些稍微不同的事情。这里有一个例子……

// Pass in the HTTPGET URL, Full Path w/Filename, and a populated Cookie Container (optional)
private async Task DownloadFileRequiringHeadersAndCookies(string getUrl, string fullPath, CookieContainer cookieContainer, CancellationToken cancellationToken)
{
    cookieContainer ??= new CookieContainer();  // TODO: FILL ME AND PASS ME IN

    using (var handler = new HttpClientHandler()
    {
        UseCookies = true,
        CookieContainer = cookieContainer, // This will, both, use the cookies passed in, and update/create cookies from the response
        ServerCertificateCustomValidationCallback = (message, cert, chain, errors) => true, // use only if it gets angry about the SSL endpoints
        AllowAutoRedirect = true,
    })
    {
        using (var client = new HttpClient(handler))
        {
            SetHeaders(client);

            using (var response = await client.GetAsync(getUrl, cancellationToken))
            {
                if (response.IsSuccessStatusCode)
                {
                    var bytes = await response.Content.ReadAsByteArrayAsync(cancellationToken);
                    await File.WriteAllBytesAsync(fullPath, bytes, cancellationToken); // This overwrites the file
                }
                else
                {
                    // TODO: HANDLE ME
                    throw new FileNotFoundException();
                }
            }
        }
    }
}

并且，要添加你需要的header…

private void SetHeaders(HttpClient client)
{
    // TODO: SET ME
    client.DefaultRequestHeaders.Connection.Add("keep-alive");
    client.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) ...");
    client.DefaultRequestHeaders.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9, ...");
    client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("gzip"));
    client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("deflate"));
    client.DefaultRequestHeaders.AcceptLanguage.Add(new StringWithQualityHeaderValue("en-US"));
    client.DefaultRequestHeaders.AcceptLanguage.Add(new StringWithQualityHeaderValue("en", .9));
    ...
}

旁白:你可以通过以下方式填充cookie容器:

循环遍历前一个响应的cookie。 这个响应可以来自HttpAgilityPack，或者WebClient，或者Puppeteer(有很多选项) 手动输入(来自配置值或硬编码值)。

你也可以在WebClient类中使用DownloadFileAsync方法。它将具有指定URI的资源下载到本地文件。此外，此方法不会阻塞调用线程。

示例:

    webClient.DownloadFileAsync(new Uri("http://www.example.com/file/test.jpg"), "test.jpg");

欲了解更多信息:

http://csharpexamples.com/download-files-synchronous-asynchronous-url-c/

using System.Net;

WebClient webClient = new WebClient();
webClient.DownloadFile("http://mysite.com/myfile.txt", @"c:\myfile.txt");