title: C# HttpClient.SendAsync always returns 404 but URL works in browser
body: <p>I am developing an C# console application for testing whether a URL is valid or not. It works well for most of URLs. But we found that there are some cases the application always got 404 response from target site but the URLs actually work in the browser. And those URLs also works when I tried them in the tools such as DHC (Dev HTTP Client). </p>
  
  <p>In the beginning, I though that this could be the reason of not adding right headers. But after tried using Fiddler to compose a http request with same headers, it works in Fiddler. </p>
  
  <p>So what's wrong with my code? Is there any bug in .NET HttpClient?</p>
  
  <p>Here are the simplified code of my test application:</p>
  
  <pre><code>class Program
  {
      static void Main(string[] args)
      {
          var urlTester = new UrlTester("http://www.hffa.it/short-master-programs/fashion-photography");
  
          Console.WriteLine("Test is started");
  
          Task.WhenAll(urlTester.RunTestAsync());
  
          Console.WriteLine("Test is stoped");
          Console.ReadKey();
      }
  
  
      public class UrlTester
      {
          private HttpClient _httpClient;
          private string _url;
  
          public UrlTester(string url)
          {
              _httpClient = new HttpClient 
              { 
                  Timeout = TimeSpan.FromMinutes(1)
              };
  
              // Add headers
              _httpClient.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36");
              _httpClient.DefaultRequestHeaders.Add("Accept-Encoding", "gzip,deflate,sdch");
              _httpClient.DefaultRequestHeaders.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8");
              _httpClient.DefaultRequestHeaders.Add("Accept-Language", "sv-SE,sv;q=0.8,en-US;q=0.6,en;q=0.4");
  
              _url = url;
          }
  
          public async Task RunTestAsync()
          {
              var httpRequestMsg = new HttpRequestMessage(HttpMethod.Get, _url);
  
              try
              {
                  using (var response = await _httpClient.SendAsync(httpRequestMsg, HttpCompletionOption.ResponseHeadersRead))
                  {
                      Console.WriteLine("Response: {0}", response.StatusCode);
                  }
              }
              catch (HttpRequestException e) 
              {
                  Console.WriteLine(e.InnerException.Message);
              }
          }
      }
  
  }
  </code></pre>