Consuming REST Web Service in .NET MVC 3

Question

I am working on a .NET 4 MVC 3 application. I'm trying to follow a domain driven design paradigm. Right now, my application is broken into two pieces, a domain and my MVC code for the web. I'd like some help in determining where in this structure I should consume a RESTful web service.

This specific project uses a RESTful web service to retrieve and persist data. In my domain, I have two entities "Customer" and "User" which pair up with web services of the same name. e.g. URL/Customer and URL/User. Each web services takes a few parameters and then returns an appropriate list of data in XML. I need to implement basic CRUD functionality in the form of (POST, GET, PUT and DELETE). Given this, I have two main questions.

1.) What type of object should I create to consume these web services? My gut instinct is to create an ICustomerService interface that defines my CRUD operations, and then create an implementation of that interface in the form of a class that uses HTTPWebConnection (or extends it?). Is there a better way to consume RESTful web services? Should this type of class be static?

2.) Where should this service code go? Again, my gut tells me that in addition to the Domain and WebUI sections of my code, I need a third, Services section that contains the interfaces and implementations of these web service clients, but since the web services are returning XML representations of the Customer and User entities that are in my domain, the services won't really be de-coupled from the domain.

Thanks in advance, Greg

EDIT

After working on various projects for some time, I've found a good way of handling REST web services in MVC.

Firstly, I create Entities that represent the various web services that I will be using. Each entity uses XML attributes to match up the properties with the XML elements. Here's a simple example for a hypothetical web service that returns information about people and their shirts (it's stupid, but the best I could come up with on the fly).

Let's say I'm getting a Person object from the web service. Here's the XML.

<result>
    <resultCount>1</resultCount>
    <person>
        <personName>Tom</personName>
        <shirt>
            <shirtColor>red</shirtColor>
            <shirtType>sweater</shirtType>
        </shirt>
    </person>
</result>

I would then have two entities: Person and Shirt. I like to include the entire class so that newbies can see everything, so I'm sorry if this is too verbose for your tastes.

Person

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Xml.Serialization;

namespace Test.Entities
{
    [XmlRoot("person")]
    public class Person
    {
        /*
        Notice that the class name doesn't match the XML Element. This is okay because we
        are using XmlElement to tell the deserializer that 
        Name and <personName> are the same thing
        */
        [XmlElement("personName")]
        public string Name { get; set; }

        [XmlElement("shirt")]
        public Shirt Shirt { get; set; }
    }
}

Shirt

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    using System.Xml.Serialization;

    namespace Test.Entities
    {
        public class Shirt
        {
            [XmlElement("shirtColor")]
            public string Color { get; set; }

            [XmlElement("shirtType")]
            public string Type { get; set; }

            /*
            This is specific to our Entity and doesn't exist in the web service so we can use
            XmlIgnore to make the deserializer ignore it
            */      
            [XmlIgnore]
            public string SpecialDbId { get; set; }
        }

    }

We can then use a XmlSerializer to convert object to XML and XML to objects. Here is a class I modified to do this. I apologize as I don't remember the original source. (There is probably a lot of room for improvement within this class)

ObjectSerializer

using System.Collections.Generic;
using System.Text;
using System.Xml;
using System.IO;
using System.Xml.Serialization;
using System;
using System.Xml.Linq;

public static class ObjectSerializer
{
    /// <summary>
    /// To convert a Byte Array of Unicode values (UTF-8 encoded) to a complete String.
    /// </summary>
    /// <param name="characters">Unicode Byte Array to be converted to String</param>
    /// <returns>String converted from Unicode Byte Array</returns>
    private static string UTF8ByteArrayToString(byte[] characters)
    {
        UTF8Encoding encoding = new UTF8Encoding();
        string constructedString = encoding.GetString(characters);
        return (constructedString);
    }

    /// <summary>
    /// Converts the String to UTF8 Byte array and is used in De serialization
    /// </summary>
    /// <param name="pXmlString"></param>
    /// <returns></returns>
    private static Byte[] StringToUTF8ByteArray(string pXmlString)
    {
        UTF8Encoding encoding = new UTF8Encoding();
        byte[] byteArray = encoding.GetBytes(pXmlString);
        return byteArray;
    }

    /// <summary>
    /// Serialize an object into an XML string
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="obj"></param>
    /// <returns></returns>
    public static string SerializeObject<T>(T obj)
    {
        try
        {
            XDocument xml;
            using (MemoryStream stream = new MemoryStream())
            {
                XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
                ns.Add("", "");
                XmlSerializer serializer = new XmlSerializer(typeof(T));
                serializer.Serialize(stream, obj, ns);
                stream.Close();
                byte[] buffer = stream.ToArray();
                UTF8Encoding encoding = new UTF8Encoding();
                string stringXml = encoding.GetString(buffer);
                xml = XDocument.Parse(stringXml);
                xml.Declaration = null;
                return xml.ToString();
            }

        }
        catch
        {
            return string.Empty;
        }
    }

    /// <summary>
    /// Reconstruct an object from an XML string
    /// </summary>
    /// <param name="xml"></param>
    /// <returns></returns>
    public static T DeserializeObject<T>(string xml)
    {
        XmlSerializer xs = new XmlSerializer(typeof(T));
        MemoryStream memoryStream = new MemoryStream(StringToUTF8ByteArray(xml));
        XmlTextWriter xmlTextWriter = new XmlTextWriter(memoryStream, Encoding.UTF8);
        return (T)xs.Deserialize(memoryStream);
    }
}

Then, create a generic service to handle your HTTP operations. I use GET and POST. Here is my class.

HttpService

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Web;
using System.Web.Mvc;
using System.Xml.Linq;

namespace Test.Infrastructure
{
    public class HttpService
    {
        public HttpService()
        {
            ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(AcceptCertificate);
        }

        public XDocument Post(Uri host, string path, Dictionary<string, string> headers, string payload, NetworkCredential credential)
        {
            try
            {
                Uri url = new Uri(host.Url, path);
                MvcHtmlString encodedPayload = MvcHtmlString.Create(payload);
                UTF8Encoding encoding = new UTF8Encoding();
                byte[] data = encoding.GetBytes(encodedPayload.ToHtmlString());

                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                request.Method = "POST";
                request.Credentials = credential;
                request.ContentLength = data.Length;
                request.KeepAlive = false;
                request.ContentType = "application/xml";

                MvcHtmlString htmlString1;
                MvcHtmlString htmlString2;
                foreach (KeyValuePair<string, string> header in headers)
                {
                    htmlString1 = MvcHtmlString.Create(header.Key);
                    htmlString2 = MvcHtmlString.Create(header.Value);
                    request.Headers.Add(htmlString1.ToHtmlString(), htmlString2.ToHtmlString());
                }

                using (Stream requestStream = request.GetRequestStream())
                {
                    requestStream.Write(data, 0, data.Length);
                    requestStream.Close();
                }

                using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
                using (Stream responseStream = response.GetResponseStream())
                {
                    if (response.StatusCode != HttpStatusCode.OK && response.StatusCode != HttpStatusCode.Created)
                    {
                        throw new HttpException((int)response.StatusCode, response.StatusDescription);
                    }

                    XDocument xmlDoc = XDocument.Load(responseStream);
                    responseStream.Close();
                    response.Close();

                    return xmlDoc;
                }
            }
            catch (Exception ex)
            {
                throw;
            }
        }

        public XDocument Get(Uri host, string path, Dictionary<string, string> parameters, NetworkCredential credential)
        {
            try
            {
                Uri url;
                StringBuilder parameterString = new StringBuilder();

                if (parameters == null || parameters.Count <= 0)
                {
                    parameterString.Clear();
                } else {
                    parameterString.Append("?");
                    foreach (KeyValuePair<string, string> parameter in parameters)
                    {
                        parameterString.Append(parameter.Key + "=" + parameter.Value + "&");
                    }
                }
                url = new Uri(host.Url, path + parameterString.ToString().TrimEnd(new char[] { '&' }));

                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                request.Credentials = credential;
                using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
                {
                    if (response.StatusCode != HttpStatusCode.OK)
                    {
                        throw new HttpException((int)response.StatusCode, response.StatusDescription);
                    }

                    XDocument xmlDoc = XDocument.Load(response.GetResponseStream());
                    return xmlDoc;

                }
            }
            catch (Exception ex)
            {
                throw;
            }
        }


        /*
        I use this class for internal web services.  For external web services, you'll want
        to put some logic in here to determine whether or not you should accept a certificate
        or not if the domain name in the cert doesn't match the url you are accessing.
        */
        private static bool AcceptCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
        {
            return true;
        }

    }
}

Then you create your repository to use the HttpService. I implemented an easy GetPeople() method that would return people from a web service query.

Repository

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.Linq;
using System.Configuration;
using Test.Entities;

namespace Test.Infrastructure
{
    public class PersonRepository
    {
        private HttpService _httpService;

        public PersonRepository()
        {
            _httpService = new HttpService();
        }

        public IQueryable<Person> GetPeople()
        {
            try
            {
                Uri host = new Uri("http://www.yourdomain.com");
                string path = "your/rest/path";
                Dictionary<string, string> parameters = new Dictionary<string, string>();

                //Best not to store this in your class
                NetworkCredential credential = new NetworkCredential("username", "password");

                XDocument xml = _httpService.Get(host, path, parameters, credential);
                return ConvertPersonXmlToList(xml).AsQueryable();

            } 
            catch
            {
                throw;
            }
        }

        private List<Person> ConvertPersonXmlToList(XDocument xml)
        {
            try
            {
                List<Person> perople = new List<Person>();
                var query = xml.Descendants("Person")
                                .Select(node => node.ToString(SaveOptions.DisableFormatting));
                foreach (var personXml in query)
                {
                    people.Add(ObjectSerializer.DeserializeObject<Person>(personXml));
                }
                return people;
            }
            catch
            {
                throw;
            }

        }
    }
}

Finally, you need to use your repository in your controller. I'm not using any dependency injection here (DI), but you'd ideally want to in your final build.

Controller

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Test.Entities;
using Test.Infrastructure;
using System.Net;
using System.Text;

namespace Test.Controllers
{
    public class PeopleController
    {
        private PersonRepository _personRepository;

        public PeopleController()
        {
            _personRepository = new PersonRepository();
        }

        public List<Person> List()
        {
            return _personRepository.GetPeople().ToList<Person>();
        }
    }
}

I typed this up on the fly and modified it from my actual solution, so I apologize for any typos or errors. I'll do my best to correct anything that I find, but this should give a good start to creating a re-usable solution for dealing with REST-based web services.

Answer 1

You're on the right track. I would place the ICustomerService in the domain package, and an HttpWebConnection implementation of this service in a separate package that references the domain package.

This class can be static, but does not have to be - if you're in doubt, then don't make it static.

You're right that the services are not completely decoupled from the domain, but that is because they implement a service contract defined in the domain layer, in terms of the domain. What is decoupled from the domain is the fact that they're soap/webservice clients or http/rest clients and those are the technical details that you don't want in your domain code.

So your service implementation translates XML into domain entities and makes them available to the other objects in the domain.