Question

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?

Answer 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.