USPSHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using System.Xml.Linq;

public class USPSAddress
    {
        public string Id { get; set; }
        public string City { get; set; }
        public string State { get; set; }
        public string Zip5 { get; set; }
    }

    public static class USPSHelper
    {

		public static USPSAddress CityStateLookup(string USPS_UserID, string zip5)
		{
			if (zip5.Length < 5)
				return null;

			XDocument requestDoc = new XDocument(
				new XElement("CityStateLookupRequest",
					new XAttribute("USERID", USPS_UserID),
					new XElement("Revision", "1"),
					new XElement("ZipCode", new XAttribute("ID", "0"), new XElement("Zip5", zip5.Substring(0, 5)))
				)
			);

			try
            {
				var url = "http://production.shippingapis.com/ShippingAPI.dll?API=CityStateLookup&XML=" + requestDoc;
				Console.WriteLine(url);
				var client = new WebClient();
				var response = client.DownloadString(url);

				var xdoc = XDocument.Parse(response.ToString());
				Console.WriteLine(xdoc.ToString());

				USPSAddress uspsAddress = xdoc.Descendants("ZipCode").Select(s => new USPSAddress
				{
					Id = GetXMLAttribute(s, "ID"),
					City = GetXMLElement(s, "City"),
					State = GetXMLElement(s, "State"),
					Zip5 = GetXMLElement(s, "Zip5")
				}).Where(x => !string.IsNullOrEmpty(x.Zip5)).FirstOrDefault();

				return uspsAddress;
			}
			catch
            {
				return null;
            }

		}

		public static string GetXMLElement(XElement element, string name)
		{
			var el = element.Element(name);
			if (el != null)
			{
				return el.Value;
			}
			return "";
		}

		public static string GetXMLAttribute(XElement element, string name)
		{
			var el = element.Attribute(name);
			if (el != null)
			{
				return el.Value;
			}
			return "";
		}

	}