xmlrpcnet with default values

Client.cs

using CookComputing.XmlRpc;
using System;

namespace XmlRpcClient
{
    class Program
    {
        static void Main(string[] args)
        {
            var service = XmlRpcProxyGen.Create<ICalcService>();

            Console.WriteLine($"1: {service.Add(1)}");
            Console.WriteLine($"1 + 2: {service.Add(1, 2)}");
            Console.WriteLine($"1 + 2 + 3: {service.Add(1, 2, 3)}");

            Console.WriteLine("XmlRpcClient. Press any key to exit");
            Console.ReadKey(true);
        }
    }

    [XmlRpcUrl("http://127.0.0.1:5678/calc")]
    public interface ICalcService : IXmlRpcProxy
    {
        [XmlRpcMethod("add")]
        int Add(int a, int b = 0, int c = 0);
    }
}

Server.cs

using CookComputing.XmlRpc;
using System;
using System.Runtime.Remoting;

namespace XmlRpcServer
{
    class Program
    {
        static void Main(string[] args)
        {
            RemotingConfiguration.Configure("XmlRpcServer.exe.config", false);
            RemotingConfiguration.RegisterWellKnownServiceType
            (
                typeof(CalcService), "calc", WellKnownObjectMode.Singleton
            );
            Console.WriteLine("XmlRpcServer. Press any key to exit");
            Console.ReadKey(true);
        }
    }

    public class CalcService : MarshalByRefObject
    {
        [XmlRpcMethod("add")]
        public int Add(int a, int b = 0, int c = 0)
        {
            return a + b + c;
        }
    }
}

XmlRpcServer.exe.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
  </startup>
  <system.runtime.remoting>
    <application>
      <channels>
        <channel ref="http" bindTo="127.0.0.1" port="5678">
          <serverProviders>
            <formatter type="CookComputing.XmlRpc.XmlRpcServerFormatterSinkProvider, CookComputing.XmlRpcServerV2" />
          </serverProviders>
        </channel>
      </channels>
    </application>
  </system.runtime.remoting>
</configuration>