处理抛出异常的（404）前的HttpWebResponse文件未找到

Question

有没有办法抛出异常的（404）之前处理HttpWebResponse.GetResponse()文件没有发现？

我有一个庞大的图像量下载，并使用在try..catch处理未发现会让表现非常糟糕文件的例外。

private bool Download(string url, string destination) 
{ 
    try 
    { 
      if (RemoteFileExists("http://www.example.com/FileNotFound.png") 
      { 
        WebClient downloader = new WebClient(); 
         downloader.DownloadFile(url, destination); 
         return true; 
      } 
    } 
    catch(WebException webEx) 
    { 
    } 
    return false; 
} 

private bool RemoteFileExists(string url) 
{ 
    try 
    { 
     //Creating the HttpWebRequest 
     HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest; 
     //Setting the Request method HEAD, you can also use GET too. 
     request.Method = "HEAD"; 
     //Getting the Web Response. 
     //Here is my question, When the image's not there, 
//the following line will throw an exception, How to avoid that? 
     HttpWebResponse response = request.GetResponse() as HttpWebResponse; 
     //Returns TURE if the Status code == 200 
     return (response.StatusCode == HttpStatusCode.OK); 
    } 
    catch 
    { 
     //Any exception will returns false. 
     return false; 
    } 
} 

Answer 1

您可以使用HttpClient，它不扔404例外：

HttpClient c = new HttpClient(); 

var resp = await c.SendAsync(new HttpRequestMessage(HttpMethod.Head, "http://www.google.com/abcde")); 

bool ok = resp.StatusCode == HttpStatusCode.OK; 

Answer 2

通过发送Async要求使用HttpClient将解决（404找不到文件）除外，所以我会将其视为这个问题的答案，但是，我想在此分享我的性能测试。我已经测试了以下方法来下载50000张图片：

方法1

try 
{ 
    // I will not check at all and let the the exception happens 
    downloader.DownloadFile(url, destination); 
    return true; 
} 
catch(WebException webEx) 
{ 
} 

方法2

try 
{ 
    // Using HttpClient to avoid the 404 exception 
    HttpClient c = new HttpClient(); 

    var resp = await c.SendAsync(new HttpRequestMessage(HttpMethod.Head, 
      "http://www.example.com/img.jpg")); 

    if (resp.StatusCode == HttpStatusCode.OK) 
    { 
     downloader.DownloadFile(url, destination); 
     return true; 
    } 
} 
catch(WebException webEx) 
{ 
} 

在我的测试中，144个图像无法获得了50.000的图像是下载，方法1中的性能比方法2快3倍。