Example of Custom Serialization in ServiceStack v4.

program.cs

using System;
using ServiceStack;
using ServiceStack.Web;

namespace CustomSerializerTest
{
	class MainClass
	{
		public static void Main()
		{
			var appHost = new AppHost(500);
			appHost.Init();
			appHost.Start("http://*:9000/");
			Console.ReadKey();
		}
	}

	public class AppHost : AppHostHttpListenerPoolBase
	{
		public AppHost(int poolSize) : base("Test Service", poolSize, typeof(TestService).Assembly)
		{
		}

		public override void Configure(Funq.Container container)
		{
			SetConfig(new HostConfig { 
				DebugMode = true,
				//DefaultContentType = "application/xml"
			});

			StreamSerializerDelegate serialize = (request, response, stream) => {
				// Dummy custom serialization method, nothing fancy
				// Replace with call to your serializer
				var o = response as IMyCustomResponse;
				if(o == null) throw new Exception("Not the expected type");
				stream.Write(string.Format("<CustomMessage Test=\"{0}\">{1}</CustomMessage>", DateTime.Now.Ticks, o.Message));
			};
			StreamDeserializerDelegate deserialize = (type, fromStream) => { throw new NotImplementedException(); };
			ContentTypes.Register("application/xml", serialize, deserialize);
		}
	}

	[FallbackRoute("/{Path*}")]
	public class RootRequest : IReturn<TestResponse>
	{
		public string Path { get; set; }
	}

	[Route("/Test", "GET")]
	public class TestRequest : IReturn<TestResponse> 
	{
	}

	public interface IMyCustomResponse
	{
		string Message { get; set; }
	}

	public class TestResponse : IMyCustomResponse
	{
		public string Message { get; set; }
	}

	public class TestService : Service
	{
		[AddHeader(ContentType = "application/xml")]
		public TestResponse Get(RootRequest request)
		{
			return new TestResponse { Message = "Hello from root" };
		}

		[AddHeader(ContentType = "application/xml")]
		public TestResponse Get(TestRequest request)
		{
			return new TestResponse { Message = "Hello from test" };
		}
	}
}