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

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.

Grx70's answer is good - just adding an alternative solution here. I ran into this problem in a Web API project where I wasn't calling SerializeObject but allowing the serialization to happen automagically.

This custom JsonConverter based on Brian Rogers' answer to a similar question did the trick for me:

public class DeepDictionaryConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (typeof(IDictionary).IsAssignableFrom(objectType) ||
                TypeImplementsGenericInterface(objectType, typeof(IDictionary<,>)));
    }

    private static bool TypeImplementsGenericInterface(Type concreteType, Type interfaceType)
    {
        return concreteType.GetInterfaces()
               .Any(i => i.IsGenericType && i.GetGenericTypeDefinition() == interfaceType);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        Type type = value.GetType();
        IEnumerable keys = (IEnumerable)type.GetProperty("Keys").GetValue(value, null);
        IEnumerable values = (IEnumerable)type.GetProperty("Values").GetValue(value, null);
        IEnumerator valueEnumerator = values.GetEnumerator();

        writer.WriteStartArray();
        foreach (object key in keys)
        {
            valueEnumerator.MoveNext();

            writer.WriteStartArray();
            serializer.Serialize(writer, key);
            serializer.Serialize(writer, valueEnumerator.Current);
            writer.WriteEndArray();
        }
        writer.WriteEndArray();
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

In my case, I was serializing a Dictionary<MyCustomType, int> property on a class where MyCustomType had properties like Name and Id. This is the result:

...
"dictionaryProp": [
    [
      {
        "name": "MyCustomTypeInstance1.Name",
        "description": null,
        "id": null
      },
      3
    ],
    [
      {
        "name": "MyCustomTypeInstance2.Name",
        "description": null,
        "id": null
      },
      2
    ]
]
...

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;