System.Net.WebException when porting from C# to F# - http

Question

I am trying to port some C# code over to F#.
The C# code has been taken from here (and slightly stripped back): https://github.com/joelpob/betfairng/blob/master/BetfairClient.cs
public bool Login(string p12CertificateLocation, string p12CertificatePassword, string username, string password)
{
var appKey = "APPKEY";
string postData = string.Format("username={0}&password={1}", username, password);
X509Certificate2 x509certificate = new X509Certificate2(p12CertificateLocation, p12CertificatePassword);
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://identitysso.betfair.com/api/certlogin");
request.UseDefaultCredentials = true;
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.Headers.Add("X-Application", appKey);
request.ClientCertificates.Add(x509certificate);
request.Accept = "*/*";
using (Stream stream = request.GetRequestStream())
using (StreamWriter writer = new StreamWriter(stream, Encoding.Default))
writer.Write(postData);
using (Stream stream = ((HttpWebResponse)request.GetResponse()).GetResponseStream())
using (StreamReader reader = new StreamReader(stream, Encoding.Default))
The C# code above works great. However, when trying to run (what I think is) F# equivalent code, without any real alterations, I get an error message.
The code is being run from the same computer, same VS installation and with exactly the same 4 arguments.
The error message I get is on the second to last line:
member x.Login(username, password,p12CertificateLocation:string, p12CertificatePassword:string) =
let AppKey = "APPKEY"
let url = "https://identitysso.betfair.com/api/certlogin"
let postData = "username=" + username + "&password=" + password
let x509certificate = new X509Certificate2(p12CertificateLocation, p12CertificatePassword)
let req = HttpWebRequest.Create(url) :?> HttpWebRequest
req.ClientCertificates.Add(x509certificate)|>ignore
req.UseDefaultCredentials <- true
req.Method <- "POST"
req.ContentType <- "application/x-www-form-urlencoded"
req.Headers.Add("X-Application",AppKey)
req.Accept <-"*/*"
use stream = req.GetRequestStream()
use writer =new StreamWriter(stream,Encoding.Default)
writer.Write(postData)
// fails on this line:
use stream = (req.GetResponse() :?> HttpWebResponse ).GetResponseStream()
// with System.Net.WebException: 'The remote server returned an error: (400) Bad Request.'
use reader = new StreamReader(stream,Encoding.Default)
I'm a bit lost here, as to my mind the two code implementations should be identical?

Answer 1

In this C# code:
using (Stream stream1 = request.GetRequestStream())
using (StreamWriter writer = new StreamWriter(stream1, Encoding.Default))
writer.Write(postData);
using (Stream stream2 = ((HttpWebResponse)request.GetResponse()).GetResponseStream())
using (StreamReader reader = new StreamReader(stream2, Encoding.Default))
writer and stream1 are flushed and closed immediately after the writer.Write call is finished, before you call request.GetResponse(). (This fact is somewhat obscured due to the, uhh.. interesting formatting of your code.)
In this F# code:
use stream1 = req.GetRequestStream()
use writer = new StreamWriter(stream1, Encoding.Default)
writer.Write(postData)
use stream2 = (req.GetResponse() :?> HttpWebResponse).GetResponseStream()
use reader = new StreamReader(stream2, Encoding.Default)
writer and stream1 stay alive and remain unflushed and unclosed when req.GetResponse() is called; you need to put them in an artificial scope to get the same behavior as C#:
do use stream1 = req.GetRequestStream()
use writer = new StreamWriter(stream1, Encoding.Default)
writer.Write(postData)
(* or
(use stream1 = req.GetRequestStream()
use writer = new StreamWriter(stream1, Encoding.Default)
writer.Write(postData))
*)
use stream2 = (req.GetResponse() :?> HttpWebResponse).GetResponseStream()
use reader = new StreamReader(stream2, Encoding.Default)

Answer 2

That's not "the C# way" to make an HTTP POST call. The typical way, in all supported .NET versions (ie 4.5.2 and later) is to use HttpClient. Even with HttpWebRequest, there are too many redundant or contradictory calls, like using default credentials (ie Windows authentication)
The C# way is this:
var client=new HttpClient("https://identitysso.betfair.com/api");
var values = new Dictionary<string, string>
{
{ "username", username },
{ "password", password }
};
var content = new FormUrlEncodedContent(values);
content.Headers.Add("X-Application",apiKey);
var response = await client.PostAsync("certlogin", content);
var responseString = await response.Content.ReadAsStringAsync();
In order to use a client certificate, you have to create the client instance using a custom HTTP Handler:
var handler = new WebRequestHandler();
var x509certificate = new X509Certificate2(certPath, certPassword);
handler.ClientCertificates.Add(certificate);
var client = new HttpClient(handler)
{
BaseAddress = new Uri("https://identitysso.betfair.com/api")
}
Writing the same code in F# is straight-forward:
let login username password (certPath:string) (certPassword:string) (apiKey:string) =
let handler = new WebRequestHandler()
let certificate = new X509Certificate2(certPath, certPassword)
handler.ClientCertificates.Add certificate |> ignore
let client = new HttpClient(handler,BaseAddress = Uri("https://identitysso.betfair.com"))
async {
let values = dict["username", username ; "password", password ]
let content = new FormUrlEncodedContent(values)
content.Headers.Add( "X-Application" ,apiKey)
let! response = client.PostAsync("api/certlogin",content) |> Async.AwaitTask
response.EnsureSuccessStatusCode() |> ignore
let! responseString = response.Content.ReadAsStringAsync() |> Async.AwaitTask
return responseString
}
The client, handler are thread safe and can be reused so they can be stored in fields. Reusing the same client means that the OS doesn't have to create a new TCP/IP connection each time, leading to improved performance. It's better to create the client separately. :
let buildClient (certPath:string) (certPassword:string) =
let handler = new WebRequestHandler()
let certificate = new X509Certificate2(certPath, certPassword)
handler.ClientCertificates.Add certificate |> ignore
new HttpClient(handler,BaseAddress = Uri("https://identitysso.betfair.com"))
let login (client:HttpClient) username password (apiKey:string) =
async {
let values = dict["username", username ; "password", password ]
let content = new FormUrlEncodedContent(values)
content.Headers.Add( "X-Application" ,apiKey)
let! response = client.PostAsync("api/certlogin",content) |> Async.AwaitTask
response.EnsureSuccessStatusCode() |> ignore
let! responseString = response.Content.ReadAsStringAsync() |> Async.AwaitTask
//Do whatever is needed here
return responseString
}