HTTP POST Returns Error: 417 "Expectation Failed."

When I try to POST to a URL it results in the following exception:

The remote server returned an error: (417) Expectation Failed.

Here's a sample code:

var client = new WebClient();

var postData = new NameValueCollection();
postData.Add("postParamName", "postParamValue");

byte[] responseBytes = client.UploadValues("http://...", postData);
string response = Encoding.UTF8.GetString(responseBytes); // (417) Expectation Failed.

Using an HttpWebRequest/HttpWebResponse pair or an HttpClient doesn't make a difference.

What's causing this exception?

Solution 1:

System.Net.HttpWebRequest adds the header 'HTTP header "Expect: 100-Continue"' to every request unless you explicitly ask it not to by setting this static property to false:

System.Net.ServicePointManager.Expect100Continue = false;

Some servers choke on that header and send back the 417 error you're seeing.

Give that a shot.

Solution 2:

Another way -

Add these lines to your application config file configuration section:

<system.net>
    <settings>
        <servicePointManager expect100Continue="false" />
    </settings>
</system.net>