I have follow step by step the tutorial available here : http://xml-rpc.net/faq/xmlrpcnetfaq.html#2.5

using CookComputing.XmlRpc;

public struct SumAndDiffValue
{
  public int sum;
  public int difference;
}

[XmlRpcUrl("http://www.cookcomputing.com/sumAndDiff.rem")]
public interface ISumAndDiff : IXmlRpcProxy
{
  [XmlRpcMethod]
  SumAndDiffValue SumAndDifference(int x, int y);
}

2.5 How do I supply authentication credentials?

ISumAndDiff proxy = (ISumAndDiff)XmlRpcProxyGen.Create(typeof(ISumAndDiff));
proxy.Credentials = new NetworkCredential("jsmith","password");
SumAndDiffValue ret = proxy.SumAndDifference(2, 3);

SumAndDiffValue ret = proxy.SumAndDifference(2, 3);

For a unknown reason doing the same thing in my api it doest not work.

The  server response was:

“This Web service require a valid user with grant access …  ”

==>  i did not supply properly the authentication and credential; therefore i add this :

For more details see:

[1] http://en.wikipedia.org/wiki/XML-RPC

In this document, I  will describes how to create a xml-rpc client using some popular programing language.

1. Server details: 

 Suppose i have a XML-RPC server that expose a Web Method call:  WMethod.CreateUser. This web method require parameters (firstname, lastname, email, login) and returns the user’s login created:

IN: string firstname,  string lastname,  string email, string login

OUT: string login