CustomErrorAnnotations.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Web.Http;
using System.Web.Http.OData;
using System.Web.Http.OData.Builder;
using Microsoft.Data.Edm;
using Microsoft.Data.OData;

namespace InstanceAnnotations
{
    class Program
    {
        static void Main(string[] args)
        {
            HttpServer server = new HttpServer();
            server.Configuration.Routes.MapODataRoute("default", "", GetModel());

            HttpClient client = new HttpClient(server);
            var response = client.GetAsync("http://localhost/Customers").Result;
            Console.WriteLine(response.Content.ReadAsStringAsync().Result);
        }

        private static IEdmModel GetModel()
        {
            ODataConventionModelBuilder builder = new ODataConventionModelBuilder();
            builder.EntitySet<Customer>("Customers");
            builder.ComplexType<CustomODataError>();
            return builder.GetEdmModel();
        }
    }

    public class Customer
    {
        public int ID { get; set; }
    }

    public class CustomODataError
    {
        public CustomODataError(string code, string message)
        {
            Code = code;
            Message = message;
        }

        public string Code { get; set; }

        public string Message { get; set; }
    }

    public class CustomersController : ODataController
    {
        public ODataError GetCustomers()
        {
            ODataError error = new ODataError();
            error.AddCustomError("Code1", "Some Custom Error1");
            error.AddCustomError("Code2", "Some Custom Error2");
            return error;
        }
    }

    public static class ODataErrorExtensions
    {
        private const string _typeName = "Collection(InstanceAnnotations.CustomODataError)";
        private const string _customErrorsAnnotationName = "ns.errors";

        public static void AddCustomError(this ODataError error, string code, string message)
        {
            ODataInstanceAnnotation annotation = error.InstanceAnnotations.Where(a => a.Name == _customErrorsAnnotationName).SingleOrDefault();
            if (annotation == null)
            {
                annotation = new ODataInstanceAnnotation(_customErrorsAnnotationName, new ODataCollectionValue { Items = new List<ODataComplexValue>(), TypeName = _typeName });
                error.InstanceAnnotations.Add(annotation);
            }

            List<ODataComplexValue> errorStrings = (annotation.Value as ODataCollectionValue).Items as List<ODataComplexValue>;
            errorStrings.Add(ToComplexValue(new CustomODataError(code, message)));
        }

        private static ODataComplexValue ToComplexValue(CustomODataError error)
        {
            ODataComplexValue complexValue = new ODataComplexValue();
            complexValue.Properties = new ODataProperty[]
            {
                new ODataProperty{ Name= "Code", Value = error.Code },
                new ODataProperty{ Name= "Message", Value = error.Message },
            };
            return complexValue;
        }
    }
}