Custom JSON.Net Dictionary serialization for when value object contains the key the dictionary uses to map to it.

CustomJsonSerializationTest.cs

// See https://unreasonablygoodsoftware.wordpress.com/2020/03/05/json-net-custom-dictionary-serialization/

using System;
using System.Collections.Generic;
using JetBrains.Annotations;
using Newtonsoft.Json;
using NUnit.Framework;
using UnityEngine.Scripting;

namespace packageNameRemoved {
	public class CustomJsonSerializationTestSimple {

		[JsonObject(MemberSerialization.OptIn)]
		private class HasCustomDict {
			[JsonProperty]
			public Dictionary<string, ClassHasString> dict;

			public HasCustomDict(IList<ClassHasString> list) {
				dict = new Dictionary<string, ClassHasString>();
				foreach (var classHasString in list) {
					dict[classHasString.id] = classHasString;
				}
			}

			[JsonConstructor]
			public HasCustomDict(Dictionary<string, ClassHasString> dict) {
				this.dict = dict;
			}

			public override string ToString() {
				return $"{GetType().Name}({nameof(dict)}:[\n  {(null != dict ? string.Join("\n  ", dict) : "null")}])";
			}
		}

		[JsonObject(MemberSerialization.OptIn)]
		private class ClassHasString {
			[JsonProperty]
			public string id;

			[JsonProperty]
			public int num;

			[JsonConstructor]
			public ClassHasString(string id, int num) {
				this.id = id;
				this.num = num;
			}

			public override string ToString() {
				return $"({nameof(id)}:{id}, {nameof(num)}:{num})";
			}
		}

		[Test]
		public void simpleSerializationWorks() {
			var hasCustomDict = new HasCustomDict(new []{
				new ClassHasString("one", 1),
				new ClassHasString("two", 2),
				new ClassHasString("three", 3),
			});

			var beforeStr = hasCustomDict.ToString();
			log("before: {0}", beforeStr);

			var json = JsonConvert.SerializeObject(hasCustomDict);
			log("JSON: {0}", json);

			var deserialized = JsonConvert.DeserializeObject<HasCustomDict>(json);
			var afterStr = deserialized.ToString();
			log("After: {0}", afterStr);

			Assert.AreEqual(beforeStr, afterStr);
		}

		[StringFormatMethod("formatStr")]
		private static void log([NotNull] string formatStr, params object[] args) {
			Console.Out.WriteLine(formatStr, args);
		}
	}

	public class CustomJsonSerializationWithListOfValues {

		[JsonObject(MemberSerialization.OptIn), Preserve]
		private class HasCustomDict {
			[JsonProperty]
			public Dictionary<string, ClassHasString> dict;

			public HasCustomDict(IList<ClassHasString> list) {
				dict = new Dictionary<string, ClassHasString>();
				foreach (var classHasString in list) {
					dict[classHasString.id] = classHasString;
				}
			}

			[JsonConstructor]
			public HasCustomDict(Dictionary<string, ClassHasString> dict) {
				this.dict = dict;
			}

			public override string ToString() {
				return $"{GetType().Name}({nameof(dict)}:[\n  {(null != dict ? string.Join("\n  ", dict) : "null")}])";
			}
		}

		[JsonObject(MemberSerialization.OptIn), Preserve]
		private class ClassHasString {
			[JsonProperty]
			public string id;

			[JsonProperty]
			public int num;

			[JsonConstructor]
			public ClassHasString(string id, int num) {
				this.id = id;
				this.num = num;
			}

			public override string ToString() {
				return $"({nameof(id)}:{id}, {nameof(num)}:{num})";
			}
		}

		private class CustomDictFormatterAsListOfValues<TKey, TValue> : JsonConverter {
			public CustomDictFormatterAsListOfValues(Func<TValue, TKey> getKeyFromValue) {
				this.getKeyFromValue = getKeyFromValue;
			}

			public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
				log("writer:{0}, value:{1} (type:{2}), serializer:{3}\n", writer, value, value.GetType(), serializer);
				writer.WriteStartArray();
				foreach (var kvp in (IDictionary<TKey, TValue>) value) {
					serializer.Serialize(writer, kvp.Value);
				}
				writer.WriteEndArray();
			}

			public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
				log("reader:{0}, objectType:{1}, existingValue:{2}, serializer:{3}\n", reader, objectType, existingValue, serializer);
				var list = serializer.Deserialize<IList<TValue>>(reader);
				var dict = new Dictionary<TKey, TValue>();
				foreach (var t in list) {
					dict[getKeyFromValue(t)] = t;
				}
				return dict;
			}

			public override bool CanWrite => true;

			public override bool CanRead => true;

			public override bool CanConvert(Type objectType) {
				var canConvert = typeof(IDictionary<TKey, TValue>).IsAssignableFrom(objectType);
				log("convert? {0} = {1}{2}", objectType, canConvert, canConvert ? "\n" : "");
				return canConvert;
			}

			private readonly Func<TValue, TKey> getKeyFromValue;
		}

		[Test]
		public void customSerializationWorksWithListOfValues() {
			var hasCustomDict = new HasCustomDict(new []{
				new ClassHasString("one", 1),
				new ClassHasString("two", 2),
				new ClassHasString("three", 3),
			});

			var beforeStr = hasCustomDict.ToString();
			log("before: {0}\n", beforeStr);

			var customDictFormatterAsListOfValues = new CustomDictFormatterAsListOfValues<string, ClassHasString>(c => c.id);
			var json = JsonConvert.SerializeObject(hasCustomDict, customDictFormatterAsListOfValues);
			log("\nJSON: {0}\n", json);

			var deserialized = JsonConvert.DeserializeObject<HasCustomDict>(json, customDictFormatterAsListOfValues);
			var afterStr = deserialized.ToString();
			log("\nAfter: {0}\n", afterStr);

			Assert.AreEqual(beforeStr, afterStr);
		}

		[StringFormatMethod("formatStr")]
		private static void log([NotNull] string formatStr, params object[] args) {
			Console.Out.WriteLine(formatStr, args);
		}
	}

}