WebClient NotFound error but working with HttpWebRequest/Response

In my WinPhone app I'm accessing a REST service. At the beginnings I was using this code:

WebClient wc = new WebClient();
wc.Credentials = credentials;
wc.Headers["App-Key"] = appKey;
wc.DownloadStringCompleted += 
    (o, args) => MessageBox.Show(args.Error == null ? "OK" : "Error");
wc.DownloadStringAsync(uri);

but it suddenly stopped working returning me a "The remote server returned an error: NotFound" error. After a google session and some clicks in the control panel, I didn't get it to work. I decided to try this other way:

HttpWebRequest request = HttpWebRequest.CreateHttp(uri);
request.Credentials = credentials;
request.Headers["App-Key"] = appKey;
request.BeginGetResponse(asResult =>
    {
        var response = request.EndGetResponse(asResult) as HttpWebResponse;
        StreamReader reader = new StreamReader(response.GetResponseStream());
        string responseString = reader.ReadToEnd();

        Dispatcher.BeginInvoke(
            () => MessageBox.Show(response.StatusCode.ToString()));
    }, null);

and it works.

I also tried to run the first snipped pointing the URI to google's home page and it works (I had to remove the credentials, of course).

Can anyone explain what's going on?

UPDATE

I managed to get it working by replacing the 开发者_运维知识库

wc.Credentials = new NetworkCredentials(username, password);

with

wc.Headers["Authorization"] = "Basic someBase64encodedString";

but i still wonder what happened and which are the differences between the first and the second line.

PS: the test URI is: https://api.pingdom.com/api/2.0/checks but you will need an app-key from them.

When using the Credentials property, the HttpWebRequest implementation will wait the challenge response from server before to send the 'Authorization' header value.

But this can be an issue in some cases, so you have to force Basic authentication by providing directly the Authorization header.

Example when using a REST Client library like Spring.Rest :

RestTemplate template = new RestTemplate("http://example.com");
template.RequestInterceptors.Add(new BasicSigningRequestInterceptor("login", "password"));
string result = template.GetForObject<string>(uri);