正确处理期望失败（417），并在WCF客户端应用程序中更改Expect100Continue

Question

My application is a C# Windows service that consumes a WCF service. When the first "Expectation failed (417)" error occurs, I change both ServicePointManager.Expect100Continue and ServicePoint.Expect100Continue to false :

try
{
    //ServicePointManager.Expect100Continue = false; // If uncomment all work
    var svc = new ServiceClient();
    svc.GetData(); // first error
}
catch (ProtocolException pex)
{
    if (pex.Message.Contains("(417)"))
    {
        ServicePointManager.Expect100Continue = false;
        var sp = ServicePointManager.FindServicePoint(new Uri(@"http://addr.to.service/service.svc"));
        sp.Expect100Continue = false;

        var svc = new ServiceClient();
        svc.GetData(); // second same error
    }
}

However, the second call to the service also fails. But if I set Expect100Continue to false before any connection, communication with the service works correctly.

Is this way correctly to handle Expect100Continue errors? I need the application adapts automatically without user action. What am I forgetting to do this work?

Answer 1

Most of the settings on ServicePointManager are treated as the default values applied on all NEW ServicePoints that are created after that point in the application's life. In the case where you change the setting after seeing the error, you are not actually changing anything on existing ServicePoint instances, including the instance associated with the connection used by WCF in this case.

In your Sample code you are calling ServicePointManager.FindServicePoint to try to find the correct ServicePoint. However, FindServicePoint has several overloads and it is easy to use that API incorrectly. For instance, FindServicePoint will try to take into account things http/https, the host you are connecting to, your proxy configuration, etc. If you are not providing the correct parameters to FindServicePoint , you can easily end up getting the wrong ServicePoint returned to you and your settings will not be applied to the ServicePoint you intended to change.

I would recommend that you use the FindServicePoint overload that takes an IWebProxy object to ensure that you get the right ServicePoint . In most cases, you should be able to pass in WebRequest.DefaultWebProxy as the IWebProxy object.

Answer 2

From the MSDN documentation of ServicePointManager.Expect100Continue,

Changing the value of this property does not affect existing ServicePoint objects. Only new ServicePoint objects created after the change are affected. Therefore changing the value on the existing WCF client will have no effect. You need to create a new WCF client, then call GetData()