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Deserializing an IEnumerable<T> with [DataContract] applied does not work

Rather new to Json.net and tried the following simple example serializing and then deserialing an object getting the error below:

using Microsoft.VisualStudio.TestTools.UnitTesting;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Text;
using System.Collections;

namespace Timehunter.Base.ServicesTests
{
    /// <summary>
    /// Summary description for JsonError
    /// </summary>
    [TestClass]
    public class JsonError
    {
  [TestMethod]
        public void TestMethod1()
        {
            JsonSerializerSettings serializerSettings = new JsonSerializerSettings()
            {
                DateFormatHandling = DateFormatHandling.IsoDateFormat,
                DateParseHandling = Newtonsoft.Json.DateParseHandling.DateTimeOffset
            };

            Act.Activities acts = new Act.Activities();
            acts.Add(new Act.Activity() { Id = 1, Name = "test1" });
            acts.Add(new Act.Activity() { Id = 2, Name = "test2" });
            string json = Newtonsoft.Json.JsonConvert.SerializeObject(acts, serializerSettings);

            Timehunter.Base.Act.Activities target = Newtonsoft.Json.JsonConvert.DeserializeObject<Timehunter.Base.Act.Activities>(json, serializerSettings);
            Assert.AreEqual("test1", target.List[0].Name, "Name of first activity");
        }
    }
}
namespace Timehunter.Base
{
    [DataContract]
    public class Activity
    {
        private int _id;
        private string _name;

        [DataMember]
        public int Id
        {
            get { return this._id; }
            set { this._id = value; }
        }
        [DataMember]
        public string Name
        {
            get { return this._name; }
            set { this._name = value; }
        }

        public Activity()
        {
            this._id = new int();
            this._name = string.Empty;
        }
    }
    [DataContract]
    public class Activities : IEnumerable<Activity>
    {
        private List<Activity> _list;
        [DataMember]
        public List<Activity> List
        {
            get { return this._list; }
            set { this._list = value; }
        }
        public Activities()
        {
            this._list = new List<Activity>();
        }

        public void Add(Activity item)
        { this._list.Add(item); }

        public bool Remove(Activity item)
        { return this._list.Remove(item); }

        public int Count()
        { return this._list.Count; }

        public IEnumerator<Activity> GetEnumerator()
        {
            return this._list.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}

And then I get the following error:

Test Name:  TestMethod1
Test FullName:  Timehunter.Base.ServicesTests.JsonError.TestMethod1
Test Source:    C:\Users\hawi.HAWCONS\Documents\Visual Studio 2015\Projects\Timehunter.Data\Timehunter.Base.ServicesTests\JsonError.cs : line 67
Test Outcome:   Failed
Test Duration:  0:00:00,2038359

Result StackTrace:  
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateNewList(JsonReader reader, JsonArrayContract contract, Boolean& createdFromNonDefaultCreator)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateList(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, Object existingValue, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
   at Timehunter.Base.ServicesTests.JsonError.TestMethod1() in C:\Users\hawi.HAWCONS\Documents\Visual Studio 2015\Projects\Timehunter.Data\Timehunter.Base.ServicesTests\JsonError.cs:line 79
Result Message: 
Test method Timehunter.Base.ServicesTests.JsonError.TestMethod1 threw exception: 
Newtonsoft.Json.JsonSerializationException: Cannot create and populate list type Timehunter.Base.Act.Activities. Path '', line 1, position 1.

What am I doing wrong?

Update 2

This is getting reverted back in 11.0.2 for backwards compatibility. Refer to the original answer for a solution.

Update

Reported as Issue #1598: DataContractAttribute does not cause JSon object serialization for IEnumerable and fixed in commit e9e2d00 . It should be in the next release after 10.0.3 which will probably be Json.NET version 11.

Original answer

I notice you have marked your Activities class with [DataContract] and [DataMember] :

[DataContract]
public class Activities : IEnumerable<Activity>
{
    private List<Activity> _list;
    [DataMember]
    public List<Activity> List
    {
        get { return this._list; }
        set { this._list = value; }
    }
    // ...
}

Applying [DataContact] will cause DataContractJsonSerializer to serialize an IEnumerable<T> as a JSON object with properties, rather than as a JSON array. Since Json.NET supports data contract attributes when applied to non-enumerables, you might be thinking that it will respect them on enumerables and collections as well.

However, it appears this is not implemented. If I serialize your class with DataContractJsonSerializer , I see

{"List":[{"Id":1,"Name":"test1"},{"Id":2,"Name":"test2"}]}

But if I serialize with Json.NET, I see that the [DataContract] was ignored:

[{"Id":1,"Name":"test1"},{"Id":2,"Name":"test2"}]

Then later it throws an exception during deserialization because it doesn't know how to add members to your IEnumerable<Activity> class. (It would have been able to add members if your class implemented ICollection<Activity> , or had a constructor with an IEnumerable<Activity> argument .)

So, should this work? The documentation page Serialization Attributes states:

The DataContractAttribute can be used as substitute for JsonObjectAttribute. The DataContractAttribute will default member serialization to opt-in.

Which implies that Json.NET ought to work the way you expect. You could report an issue about it if you want -- at least the documentation should be clarified.

As a workaround , if you want to force Json.NET to serialize a collection as an object, you need to use [JsonObject] instead:

[DataContract]
[JsonObject(MemberSerialization = MemberSerialization.OptIn)]
public class Activities : IEnumerable<Activity>
{
    private List<Activity> _list;

    [DataMember]
    [JsonProperty]
    public List<Activity> List
    {
        get { return this._list; }
        set { this._list = value; }
    }

    // Remainder unchanged.
}

If you have many enumerable classes with [DataContract] applied, or cannot add a dependency on Json.NET to your models, you can create a custom ContractResolver that checks for the presence of [DataContract] on enumerable classes and serializes them as objects:

public class DataContractForCollectionsResolver : DefaultContractResolver
{
    // As of 7.0.1, Json.NET suggests using a static instance for "stateless" contract resolvers, for performance reasons.
    // http://www.newtonsoft.com/json/help/html/ContractResolver.htm
    // http://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Serialization_DefaultContractResolver__ctor_1.htm
    // "Use the parameterless constructor and cache instances of the contract resolver within your application for optimal performance."
    static DataContractForCollectionsResolver instance;

    static DataContractForCollectionsResolver() { instance = new DataContractForCollectionsResolver(); }

    public static DataContractForCollectionsResolver Instance { get { return instance; } }

    protected DataContractForCollectionsResolver() : base() { }

    protected override JsonContract CreateContract(Type objectType)
    {
        var t = (Nullable.GetUnderlyingType(objectType) ?? objectType);
        if (!t.IsPrimitive 
            && t != typeof(string)
            && !t.IsArray
            && typeof(IEnumerable).IsAssignableFrom(t) 
            && !t.GetCustomAttributes(typeof(JsonContainerAttribute),true).Any()) 
        { 
            if (t.GetCustomAttributes(typeof(DataContractAttribute),true).Any()) 
                return base.CreateObjectContract(objectType);
        }
        return base.CreateContract(objectType);
    }
}

Then use the following settings:

var serializerSettings = new JsonSerializerSettings()
{
    DateFormatHandling = DateFormatHandling.IsoDateFormat,
    DateParseHandling = Newtonsoft.Json.DateParseHandling.DateTimeOffset,
    ContractResolver = DataContractForCollectionsResolver.Instance
};

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