The request failed with HTTP status 417: Expectation failed

When using WebServices and calling some Web Methods, you may come across this exception:

The request failed with HTTP status 417: Expectation failed.

When Expect 100-Continue property is set to true (this is actually set within part of the .NET Framework, which makes this a difficult problem to overcome. You have to override this in more than one place), this 417 error occurs when the proxy server doesn't support 100-Continue.

In the case of expect 100-Continue, the client will expect to receive a 100-Continue response from the server to indicate that the client should send the data to be posted. This mechanism allows clients to avoid sending large amounts of data over the network when the server, based on the request headers, intends to reject the request.

The Expect 100-Continue behavior is fully described in IETF RFC 2616 Section 10.1.1.

You can also read more about it in MSDN.

To work around this exception, you can do a simple thing: Disable expect 100 continue.

So you may ask how to disable expect 100 continue?

It is easy. Here are two alternative ways:

1. We need to have this line of code before making any web requests:

   System.Net.ServicePointManager.Expect100Continue = false; 

2. Add these lines to the application's configuration file (between <configuration> and </configuration>):

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

While this workaround is nice, I think a better long term solution is to upgrade/replace the proxy server to handle 100-continue calls.

So far in this short tip, we've learned what is Expect 100, how it is caused and how to workaround this expectation.

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