MS Dynamics CRM - Display Unique/Auto Number on the form before save.

My client requirement is display the unique reg number for new contact before the save record.
My Previous blog <a href="http://makdns.blogspot.com/2016/03/ms-dynamics-crm-generate-uniqueauto.html">MS Dynamics CRM - Generate Unique/Auto Number</a> will show the reg number on form after save the new contact. if the user clicks save and close, they can't see the number.

So i come up with the web service concept to generate unique number for new contact and display on the contact screen.
Also i am using SQL Server sys.sequences for generating new number.

Web Service Code

[ServiceContract]
public interface IVoterSearch
{
[OperationContract]
[WebInvoke(Method = "POST",
    RequestFormat = WebMessageFormat.Json,
    ResponseFormat = WebMessageFormat.Xml,
    BodyStyle = WebMessageBodyStyle.WrappedRequest)]
string RegNumber(string userGuid);
}

/*
 * Generate new reg number
    --https://msdn.microsoft.com/en-us/library/ff878091.aspx
    --drop SEQUENCE dbo.DNSContactRegNumber
    --select * from sys.sequences
    --CREATE SEQUENCE dbo.DNSContactRegNumber
--    AS int
--   START WITH 123456
--    INCREMENT BY 1;
    --SELECT NEXT VALUE FOR dbo.DNSContactRegNumber As NewRegNumber;
*/
public string RegNumber(string userGuid)
{
    //checking the user is login dynamics crm user for security purpose.
    var valid = IsUserValid(userGuid, crmDB);
    if (!(valid == string.Empty)) return valid;
    var regid = new XDocument();
    string NewRegNumber = "0";
    string dbExtCon = System.Configuration.ConfigurationManager.ConnectionStrings["DNS_EXT_DB_ConnectionString"].ConnectionString;
    try
    {
using (SqlConnection Con = new SqlConnection(dbCon))
{
   Con.Open();
   SqlCommand cmd = new SqlCommand();
   cmd.CommandText = "SELECT NEXT VALUE FOR dbo.DNSContactRegNumber As NewRegNumber";
   cmd.Connection = Con;
   SqlDataReader dr = cmd.ExecuteReader();

   while (dr.Read())
   {
NewRegNumber = dr["NewRegNumber"].ToString();
   }
   dr.Close();
}


regid = new XDocument(
   new XElement("Contact",
new XAttribute("Error", "0"),
new XAttribute("Message", ""),
new XAttribute("NewRegNumber", NewRegNumber)
   )
);
    }
    catch (Exception e)
    {
regid = new XDocument(
new XElement("Contact",
   new XAttribute("Error", "1"),
   new XAttribute("Message", e.Message.ToString()),
   new XAttribute("NewRegNumber", NewRegNumber)
)
   );
    }
    return regid.ToString();
}

//verify the login user is valid active system user.
private string IsUserValid(string userGuid, DNSCRMDBDataContext db)
{
    var exists = false;
    loginUserGuid = loginUserGuid.Replace("}", "").Replace("{", "");
    var valid = db.SystemUsers
  .Where(u => u.SystemUserId.ToString() == userGuid)
  .Select(u => u.IsDisabled)
  .FirstOrDefault().ToString();
    if (valid == string.Empty) exists = false;
    else exists = !bool.Parse(valid);

    if (!exists)
    {
var message = new XDocument( new XElement("List",
new XAttribute("Error", "1"),
new XAttribute("Message", "Invalid CRM USER")
));
return message.ToString();
    }

    return string.Empty;
}

Contact Form Onload JavaScript Code, calling the web service on form load event and assign new reg number.

/*
assign reg number when form is opened
Create New Reg Number & assign to the field.
*/
function NewRegNumber() {
var userId = Xrm.Page.context.getUserId();
var svcURL = location.protocol + '//' + location.hostname + (location.port ? ':' + location.port : '') + "/DNS.VM.Service/";
$.ajax({
   type: "POST",
   url: svcURL + "Contact.svc/RegNumber",
   async: true,
   dataType: "xml",
   contentType: "application/json; charset=utf-8",
   data: JSON.stringify({ userGuid: userId}),
   success: function (xml) {
xml = $.parseXML($(xml).find("string").text());
$(xml).find("Contact").each(function () {
   var error = $(this).attr('Error');
   var regnumber = $(this).attr('NewRegNumber');
   var message = $(this).attr('Message');
   if (error == "1")
    alert(message);
   else
    Xrm.Page.data.entity.attributes.get("dns_regnumber").setValue(regnumber);
});
   },
   error: function (XMLHttpRequest, textStatus, errorThrown) {
alert(textStatus);
   }
});
}