Flurl.Http PostMultipartAsync caused some errors - flurl

Question

I'm posting some data to a server. This piece of code works fine a few months ago. But now it threw “Flurl.Http.FlurlHttpException”.
Flurl.Http 2.3.2
string res = "";
try
{
var obj = new { api_key = "key", api_secret = "secret", start = 1 };
res = await "https://api-cn.faceplusplus.com/facepp/v3/faceset/getfacesets"
.PostMultipartAsync(mp => mp
.AddStringParts(obj)
).ReceiveString();
}
catch (FlurlHttpException ex)
{
Debug.WriteLine(ex.Message);
}
The only difference is Win10 version, from 1803 to 1809.
I changed to use "PostUrlEncodedAsync", the server returns success.
But this method can't add a file.
res = await apiUrl
.PostUrlEncodedAsync(obj)
.ReceiveString();
Here is the FlurlHttpException:
{Flurl.Http.FlurlHttpException: Call failed with status code 400 (): POST https://api-cn.faceplusplus.com/facepp/v3/faceset/getfacesets
at Flurl.Http.FlurlRequest.<HandleExceptionAsync>d__23.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
at Flurl.Http.FlurlRequest.<SendAsync>d__19.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Flurl.Http.FlurlRequest.<SendAsync>d__19.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Flurl.Http.HttpResponseMessageExtensions.<ReceiveString>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at FacePlusPlusUWP.FaceServiceClient.<SendRequestAsync>d__26.MoveNext()}

Answer 1

As indicated by FlurlHttpException.Message, you're getting a 400 (Bad Request) response from the server. I am not familiar with the API you're using, but the first thing I would check is whether the server sent you a helpful response body. You can do that in your catch block like this:
catch (FlurlHttpException ex)
{
var resp = await ex.GetResponseStringAsync();
Debug.WriteLine(resp);
}