Provider Hosted high trust App- 400 Bad request

Question

I am getting 400 bad request when provider hosted app is trying to connect to SharePoint.Below is the stacktrace:

System.Net.WebException: The remote server returned an error: (400) Bad Request.
   at System.Net.HttpWebRequest.GetResponse()
   at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute()
   at Microsoft.SharePoint.Client.ClientRequest.ExecuteQueryToServer(ChunkStringBuilder sb)
   at Project.SharePoint.Web.Controllers.HomeController.GetLoggedInUserName()

The server logs shows that /_vti_bin/client.svc/ProcessQuery is giving me 400 bad request.

I have tried to connect to the client.svc from browser of the server where the MVC provider is installed and it works, so there is no access issue. Any help would be great.

Edit:

[SharePointContextFilter]
        public string GetLoggedInUserName()
        {
            string userName = string.Empty;

                User spUser = null;

                var spContext = SharePointContextProvider.Current.GetSharePointContext(HttpContext);

                using (var clientContext = spContext.CreateUserClientContextForSPHost())
                {
                    if (clientContext != null)
                    {
                        var web = clientContext.Web;
                        spUser = web.CurrentUser;
                        clientContext.Load(spUser, user => user.Title);
                        clientContext.ExecuteQuery();
                        userName = spUser.Title;
                    }
                }

            return userName;
        }

Answer 1

To debug this, I would suggest that you setup Fiddler on the Remote Web, and then modify the web.config to have these lines (make sure to take these out when you are done debugging)

<system.net>
  <defaultProxy>
    <proxy  proxyaddress="http://127.0.0.1:8888" />      
  </defaultProxy>
</system.net>

This will make Fiddler a proxy for your web application, so that you can see the OAuth2 and ProcessQuery calls.

With Fiddler running, access the problem page. Then, go to Fiddler, and find a url that looks like this: /vti_bin/client.svc/ProcessQuery

Click on this url in Fiddler, and then click on the "Inspectors" tab on the top right. You should see a Request that was sent as XML and a response sent in JSON (you might want to start with the "RAW" view, then switch to "XML" and "JSON".

In the JSON response, look to see if there is an "error" and also an "error_description". Hopefully these will give you clues as to what went wrong. If not, try looking at some of the previous requests/responses in Fiddler (e.g. /vti/bin/client.svc) to see if there are any errors there.