How can I serialize/deserialize a dictionary with custom keys using Json.Net?

Question

I have the following class, that I use as a key in a dictionary:

    public class MyClass
    {
        private readonly string _property;

        public MyClass(string property)
        {
            _property = property;
        }

        public string Property
        {
            get { return _property; }
        }

        public override bool Equals(object obj)
        {
            MyClass other = obj as MyClass;
            if (other == null) return false;
            return _property == other._property;
        }

        public override int GetHashCode()
        {
            return _property.GetHashCode();
        }
    }

The test I am running is here:

    [Test]
    public void SerializeDictionaryWithCustomKeys()
    {
        IDictionary<MyClass, object> expected = new Dictionary<MyClass, object>();
        expected.Add(new MyClass("sth"), 5.2);
        JsonSerializerSettings jsonSerializerSettings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.All };
        string output = JsonConvert.SerializeObject(expected, Formatting.Indented, jsonSerializerSettings);
        var actual = JsonConvert.DeserializeObject<IDictionary<MyClass, object>>(output, jsonSerializerSettings);
        CollectionAssert.AreEqual(expected, actual);
    }

The test fails, because Json.Net seems to be using the ToString() method on the dictionary keys, instead of serializing them properly. The resulting json from the test above is:

{
  "$type": "System.Collections.Generic.Dictionary`2[[RiskAnalytics.UnitTests.API.TestMarketContainerSerialisation+MyClass, RiskAnalytics.UnitTests],[System.Object, mscorlib]], mscorlib",
  "RiskAnalytics.UnitTests.API.TestMarketContainerSerialisation+MyClass": 5.2
}

which is clearly wrong. How can I get it to work?

Answer 1

Simpler, full solution, using a custom JsonConverter

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;

public class CustomDictionaryConverter<TKey, TValue> : JsonConverter
{
    public override bool CanConvert(Type objectType) => objectType == typeof(Dictionary<TKey, TValue>);

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        => serializer.Serialize(writer, ((Dictionary<TKey, TValue>)value).ToList());

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        => serializer.Deserialize<KeyValuePair<TKey, TValue>[]>(reader).ToDictionary(kv => kv.Key, kv => kv.Value);
}

Usage:

[JsonConverter(typeof(CustomDictionaryConverter<KeyType, ValueType>))]
public Dictionary<KeyType, ValueType> MyDictionary;

Answer 2

This should do the trick:

Serialization:

JsonConvert.SerializeObject(expected.ToArray(), Formatting.Indented, jsonSerializerSettings);

By calling expected.ToArray() you're serializing an array of KeyValuePair<MyClass, object> objects rather than the dictionary.

Deserialization:

JsonConvert.DeserializeObject<KeyValuePair<IDataKey, object>[]>(output, jsonSerializerSettings).ToDictionary(kv => kv.Key, kv => kv.Value);

Here you deserialize the array and then retrieve the dictionary with .ToDictionary(...) call.

I'm not sure if the output meets your expectations, but surely it passes the equality assertion.