我有精细的工作，直到一两天以前C＃程序。我用下面的代码片段抢页面：

I have a C# program which worked fine until a day or two ago. I use the following snippet to grab a page:

string strSiteListPath = @"http://www.ngs.noaa.gov/CORS/dates_sites.txt";
Uri uriSiteListPath = new Uri(strSiteListPath);
System.Net.WebClient oWebClient = new System.Net.WebClient();
strStationList = oWebClient.DownloadString(uriSiteListPath);

但它始终会返回一个404 Not Found错误。该网页完全存在，欢迎您自己尝试一下。因为它的工作日前，并没有在我的code变了，我给想也许在某些方面改变了web服务器。这很好，它会发生，但究竟曾在这里发生了什么？

But it consistently returns a 404 Not Found error. That page completely exists, you are welcome to try it yourself. Because it worked days ago, and nothing in my code changed, I am given to think maybe the web-server changed in some way. That's fine, it'll happen, but what exactly has happened here?

为什么我可以浏览到该文件手动，但DownloadString无法获取文件？

Why can I browse to the file manually, but DownloadString fails to get the file?

编辑：

为了完整起见，code现在的样子：

For completeness, the code now looks like:

string strSiteListPath = @"http://www.ngs.noaa.gov/CORS/dates_sites.txt";
Uri uriSiteListPath = new Uri(strSiteListPath);

System.Net.WebClient oWebClient = new System.Net.WebClient();
oWebClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:25.0) Gecko/20100101 Firefox/25.0");

strStationList = oWebClient.DownloadString(uriSiteListPath);

再次感谢，托马斯Levesque的！

Thanks again, Thomas Levesque!

推荐答案

显然，该网站需要你有一个有效的用户代理头。如果设置了头这样的事情：

Apparently the site requires that you have a valid User-Agent header. If you set that header to something like that:

Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:25.0) Gecko/20100101 Firefox/25.0

然后请求正常工作。

Then the request works fine.