WebClient.DownloadString() 返回带有特殊字符的字符串

Question

I have an issue with some content that we are downloading from the web for a screen scraping tool that I am building.

in the code below, the string returned from the web client download string method returns some odd characters for the source download for a few (not all) web sites.

I have recently added http headers as below. Previously the same code was called without the headers to the same effect. I have not tried variations on the 'Accept-Charset' header, I don't know much about text encoding other than the basics.

The characters, or character sequences that I refer to are:

"  "

and

" Â "

These characters are not seen when you use "view source" in a web browser. What could be causing this and how can I rectify the problem?

string urlData = String.Empty;
WebClient wc = new WebClient();

// Add headers to impersonate a web browser. Some web sites 
// will not respond correctly without these headers
wc.Headers.Add("User-Agent", "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2.12) Gecko/20101026 Firefox/3.6.12");
wc.Headers.Add("Accept", "*/*");
wc.Headers.Add("Accept-Language", "en-gb,en;q=0.5");
wc.Headers.Add("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");

urlData = wc.DownloadString(uri);

Answer 1

 is the windows-1252 representation of the octets EF BB BF . That's the UTF-8 byte-order marker , which implies that your remote web page is encoded in UTF-8 but you're reading it as if it were windows-1252. According to the docs , WebClient.DownloadString uses Webclient.Encoding as its encoding when it converts the remote resource into a string. Set it to System.Text.Encoding.UTF8 and things should theoretically work.

Answer 2

The way WebClient.DownloadString is implemented is very dumb. It should get the character encoding from the Content-Type header in the response, but instead it expects the developer to tell the expected encoding beforehand. I don't know what the developers of this class were thinking.

I have created an auxiliary class that retrieves the encoding name from the Content-Type header of the response:

public static class WebUtils
{
    public static Encoding GetEncodingFrom(
        NameValueCollection responseHeaders,
        Encoding defaultEncoding = null)
    {
        if(responseHeaders == null)
            throw new ArgumentNullException("responseHeaders");

        //Note that key lookup is case-insensitive
        var contentType = responseHeaders["Content-Type"];
        if(contentType == null)
            return defaultEncoding;

        var contentTypeParts = contentType.Split(';');
        if(contentTypeParts.Length <= 1)
            return defaultEncoding;

        var charsetPart =
            contentTypeParts.Skip(1).FirstOrDefault(
                p => p.TrimStart().StartsWith("charset", StringComparison.InvariantCultureIgnoreCase));
        if(charsetPart == null)
            return defaultEncoding;

        var charsetPartParts = charsetPart.Split('=');
        if(charsetPartParts.Length != 2)
            return defaultEncoding;

        var charsetName = charsetPartParts[1].Trim();
        if(charsetName == "")
            return defaultEncoding;

        try
        {
            return Encoding.GetEncoding(charsetName);
        }
        catch(ArgumentException ex) 
        {
            throw new UnknownEncodingException(
                charsetName,   
                "The server returned data in an unknown encoding: " + charsetName, 
                ex);
        }
    }
}

( UnknownEncodingException is a custom exception class, feel free to replace for InvalidOperationException or whatever else if you want)

Then the following extension method for the WebClient class will do the trick:

public static class WebClientExtensions
{
    public static string DownloadStringAwareOfEncoding(this WebClient webClient, Uri uri)
    {
        var rawData = webClient.DownloadData(uri);
        var encoding = WebUtils.GetEncodingFrom(webClient.ResponseHeaders, Encoding.UTF8);
        return encoding.GetString(rawData);
    }
}

So in your example you would do:

urlData = wc.DownloadStringAwareOfEncoding(uri);

...and that's it.

Answer 3

var client = new WebClient { Encoding = System.Text.Encoding.UTF8 };

var json = client.DownloadString(url);

Answer 4

in my case , i deleted ever header related to language ,charset etc EXcept user agent and cookie . it worked..

 // try commenting
 //wc.Headers.Add("Accept-Language", "en-gb,en;q=0.5");
 //wc.Headers.Add("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");

Answer 5

None of them didn't work for me for some special websites such as "www.yahoo.com". The only way which I resolve my problem was changing DownloadString to OpenRead and using UserAgent header like sample code. However, a few sites like "www.varzesh3.com" didn't work with any of methods!

WebClient client = new WebClient()    
client.Headers.Add(HttpRequestHeader.UserAgent, "");
var stream = client.OpenRead("http://www.yahoo.com");
StreamReader sr = new StreamReader(stream);
s = sr.ReadToEnd();

Answer 6

You guys rock! I've been trying to Invoke-WebRequest an AWS URL with an embedded security token. I haven't been able to download it for the life of me. Finally after adding the Accept and User-Agent displayed in this thread to my headers, I was finally able to download the json objects. Thank you!