moodle api rest call in c# -

i'm doing next api rest call in c#:

string token = "e2a35dbfdaee78097c7ba489xxxxxxxx";   moodleuser user = new moodleuser();  user.username = webutility.urlencode("username");  user.password = webutility.urlencode("the_password");  user.firstname = webutility.urlencode("michael");  user.lastname = webutility.urlencode("york");  user.email = webutility.urlencode("test@gmail.com");  list<moodleuser> userlist = new list<moodleuser>(); userlist.add(user);  array arrusers = userlist.toarray();  string postdata = string.format(@"users[0][username]={0}&users[0][password]={1}&users[0][firstname]={2}&users[0][lastname]={3}&users[0][email]={4}&users[0][preferences][0][type]={5}&users[0][preferences][0][value]={6}", user.username, user.password, user.firstname, user.lastname, user.email, **"auth_forcepasswordchange", "1");**   string createrequest = string.format("http://domain.es/webservice/rest/server.php?wstoken={0}&wsfunction={1}&moodlewsrestformat=json", token, "core_user_create_users");  // call moodle rest service  httpwebrequest req = (httpwebrequest)webrequest.create(createrequest); req.method = "post"; req.contenttype = "application/x-www-form-urlencoded";  // encode parameters form data:  byte[] formdata = utf8encoding.utf8.getbytes(postdata); req.contentlength = formdata.length;   // write out form data request: using (stream post = req.getrequeststream())  {       post.write(formdata, 0, formdata.length);  }  .... ..... 

the problem occurs when call api preferences array auth_forcepasswordchange, result of errror "invalid_parameter_exception".

when call without preferences parameter, works perfectly.

tank much.

the code below work getting course information. generated token in moodle , use token in api call

string result = "";         string methodname = "getcourse";                           string apiname = "core_course_get_courses";                    string apicall = moodleurl + "?wstoken=" + token + "&wsfunction=" + apiname + "&moodlewsrestformat=json";          try         {             using (webclient client = new webclient())             {                 client.baseaddress = apicall;                 client.headers[httprequestheader.accept] = "application/json";                 result = client.downloadstring(apicall);                 validatejson validate = new validatejson();