EndpointData

EndpointData.cs

using System.Net;

namespace JustEat.StatsD.EndpointLookups
{
    public class EndpointData
    {
        private readonly IPEndPoint _ipEndpoint;

        private readonly string _hostName;
        private readonly int _port;

        public EndpointData(string hostNameOrAddress, int port)
        {
            IPAddress address;
            if (IPAddress.TryParse(hostNameOrAddress, out address))
            {
                //if we were given an IP instead of a hostname, 
                //we can happily cache it off for the life of this class
                _ipEndpoint = new IPEndPoint(address, _port);
                _hostName = null;
                _port = 0;
            }
            else
            {
                _hostName = hostNameOrAddress;
                _port = port;
                _ipEndpoint = null;
            }
        }

        // todo- decide 
        // inject the IDnsEndpointMapper here or constructor?,
        // or fold that code in here?
        // this might need to be async as there is a ".Result" inside GetIPEndPoint
        public IPEndPoint GetEndpoint(IDnsEndpointMapper mapper)
        {
            if (_ipEndpoint != null)
            {
                return _ipEndpoint;
            }

            // Only DNS resolve if we were given a hostname
            return mapper.GetIPEndPoint(_hostName, _port); 
        } 
    }
}

EndpointFactory.cs

// the functional approach is less code.
// and it only closes over the parts that it uses
// what you need is a way to get the IPEndPoint as it is now, 
// so this can be typed as  Func<IPEndPoint>, i.e. an IPEndPoint factory.
// and a factory to build that factory
public static class EnpointFactoryFactory
{
    public static Func<IPEndPoint> ManufactureEndpointFactory(string hostNameOrAddress, int port, IDnsEndpointMapper mapper)
    {
        IPAddress address;
        if (IPAddress.TryParse(hostNameOrAddress, out address))
        {
            //if we were given an IP instead of a hostname, 
            //we can happily cache it off for the life of this class
            var ipEndpoint = new IPEndPoint(address, port);
            return () => ipEndpoint;
        }

        // Only DNS resolve if we were given a hostname
        // still have the problem that the "GetIPEndPoint" is potentially async
        return () => mapper.GetIPEndPoint(hostNameOrAddress, port);
    }
}