use the xml-rpc.net library

It's very simple to use the xml-rpc.net library, here is what you need to do:

[XmlRpcUrl("http://url_to_your_server/api.php")]
public interface ISumAndDiff : IXmlRpcProxy
{
    [XmlRpcMethod("your.remote.procedure")]
    string testMyClient(string test);
}

ISumAndDiff proxy = XmlRpcProxyGen.Create<ISumAndDiff>();

string ret = proxy.testMyClient("test");