Another failure at deserializing data with discriminated unions, in F# - serialization

Question

Following a question where the answer provided a working solution to serialize / deserialize discriminated unions (IgnoreMissingMember setting doesn't seem to work with FSharpLu.Json deserializer)
I have now a practical case where this fails (although it works in simpler cases).
here is the test code:
open System.Collections.Generic
open Microsoft.FSharpLu.Json
open Newtonsoft.Json
open Newtonsoft.Json.Serialization
// set up the serialization / deserialization based on answer from:
// https://stackoverflow.com/questions/62364229/ignoremissingmember-setting-doesnt-seem-to-work-with-fsharplu-json-deserializer/62364913#62364913
let settings =
JsonSerializerSettings(
NullValueHandling = NullValueHandling.Ignore,
Converters = [| CompactUnionJsonConverter(true, true) |]
)
let serialize object =
JsonConvert.SerializeObject(object, settings)
let deserialize<'a> object =
JsonConvert.DeserializeObject<'a>(object, settings)
// define the type used
type BookSide =
| Bid
| Ask
type BookEntry =
{
S : float
P : float
}
type BookSideData =
Dictionary<int, BookEntry>
type BookData =
{
Data: Dictionary<BookSide, BookSideData>
}
static member empty =
{
Data = Dictionary<BookSide, BookSideData>(dict [ (BookSide.Bid, BookSideData()); (BookSide.Ask, BookSideData()) ])
}
// make some sample data
let bookEntry = { S=3.; P=5. }
let bookData = BookData.empty
bookData.Data.[BookSide.Bid].Add(1, bookEntry)
// serialize. This part works
let s = serialize bookData
// deserialize. This part fails
deserialize<BookData> s
the serialized test data will look like this:
{"Data":{"Bid":{"1":{"S":3.0,"P":5.0}},"Ask":{}}}
but deserializing will crash like this:
Could not convert string 'Bid' to dictionary key type 'FSI_0023+BookSide'. Create a TypeConverter to convert from the string to the key type object.
although the serialization / deserialization of the DU through FSharpLu which has a DU converter.
The reason I am trying to find some automated solution, vs writing a custom TypeConverter (besides the fact I've never done it) is that I have a lot of types I do not control to go through.
Here is a fiddle:
https://dotnetfiddle.net/Sx0k4x

Answer 1

Your basic problem is that you are using BookSide as a dictionary key -- but this is an f# union which makes it a complex key -- one not immediately convertible to and from a string. Unfortunately Json.NET does not support complex dictionary keys out of the box as is stated in its Serialization Guide:
When serializing a dictionary, the keys of the dictionary are converted to strings and used as the JSON object property names. The string written for a key can be customized by either overriding ToString() for the key type or by implementing a TypeConverter. A TypeConverter will also support converting a custom string back again when deserializing a dictionary.
There are two basic approaches to handling this issue:
Implement a TypeConverter as is shown in, e.g., Not ableTo Serialize Dictionary with Complex key using Json.net.
Serialize the dictionary as an array of key/value pair objects e.g. as is shown in Serialize dictionary as array (of key value pairs).
Since your data model includes dictionaries with a variety of keys (DU, strings and ints) the second solution would appear to be the only possibility. The following DictionaryConverter should have the necessary logic:
let inline isNull (x:^T when ^T : not struct) = obj.ReferenceEquals (x, null)
type Type with
member t.BaseTypesAndSelf() =
t |> Seq.unfold (fun state -> if isNull state then None else Some(state, state.BaseType))
member t.DictionaryKeyValueTypes() =
t.BaseTypesAndSelf()
|> Seq.filter (fun i -> i.IsGenericType && i.GetGenericTypeDefinition() = typedefof<Dictionary<_,_>>)
|> Seq.map (fun i -> i.GetGenericArguments())
type JsonReader with
member r.ReadAndAssert() =
if not (r.Read()) then raise (JsonReaderException("Unexpected end of JSON stream."))
r
member r.MoveToContentAndAssert() =
if r.TokenType = JsonToken.None then r.ReadAndAssert() |> ignore
while r.TokenType = JsonToken.Comment do r.ReadAndAssert() |> ignore
r
type internal DictionaryReadOnlySurrogate<'TKey, 'TValue>(i : IDictionary<'TKey, 'TValue>) =
interface IReadOnlyDictionary<'TKey, 'TValue> with
member this.ContainsKey(key) = i.ContainsKey(key)
member this.TryGetValue(key, value) = i.TryGetValue(key, &value)
member this.Item with get(index) = i.[index]
member this.Keys = i.Keys :> IEnumerable<'TKey>
member this.Values = i.Values :> IEnumerable<'TValue>
member this.Count = i.Count
member this.GetEnumerator() = i.GetEnumerator()
member this.GetEnumerator() = i.GetEnumerator() :> IEnumerator
type DictionaryConverter () =
// ReadJson adapted from this answer https://stackoverflow.com/a/28633769/3744182
// To https://stackoverflow.com/questions/28451990/newtonsoft-json-deserialize-dictionary-as-key-value-list-from-datacontractjsonse
// By https://stackoverflow.com/users/3744182/dbc
inherit JsonConverter()
override this.CanConvert(t) = (t.DictionaryKeyValueTypes().Count() = 1) // If ever implemented for IReadOnlyDictionary<'TKey, 'TValue> then reject DictionaryReadOnlySurrogate<'TKey, 'TValue>
member private this.ReadJsonGeneric<'TKey, 'TValue> (reader : JsonReader, t : Type, existingValue : obj, serializer : JsonSerializer) : obj =
let contract = serializer.ContractResolver.ResolveContract(t)
let dict = if (existingValue :? IDictionary<'TKey, 'TValue>) then existingValue :?> IDictionary<'TKey, 'TValue> else contract.DefaultCreator.Invoke() :?> IDictionary<'TKey, 'TValue>
match reader.MoveToContentAndAssert().TokenType with
| JsonToken.StartArray ->
let l = serializer.Deserialize<List<KeyValuePair<'TKey, 'TValue>>>(reader)
for p in l do dict.Add(p)
dict :> obj
| JsonToken.StartObject ->
serializer.Populate(reader, dict)
dict :> obj
| JsonToken.Null -> null // Or throw an exception if you prefer
| _ -> raise (JsonSerializationException(String.Format("Unexpected token {0}", reader.TokenType)))
override this.ReadJson(reader, t, existingValue, serializer) =
let keyValueTypes = t.DictionaryKeyValueTypes().Single(); // Throws an exception if not exactly one.
let m = typeof<DictionaryConverter>.GetMethod("ReadJsonGeneric", BindingFlags.NonPublic ||| BindingFlags.Instance ||| BindingFlags.Public);
m.MakeGenericMethod(keyValueTypes).Invoke(this, [| reader; t; existingValue; serializer |])
member private this.WriteJsonGeneric<'TKey, 'TValue> (writer : JsonWriter, value : obj, serializer : JsonSerializer) =
let dict = value :?> IDictionary<'TKey, 'TValue>
let keyContract = serializer.ContractResolver.ResolveContract(typeof<'Key>)
// Wrap the value in an enumerator or read-only surrogate to prevent infinite recursion.
match keyContract with
| :? JsonPrimitiveContract -> serializer.Serialize(writer, new DictionaryReadOnlySurrogate<'TKey, 'TValue>(dict))
| _ -> serializer.Serialize(writer, seq { yield! dict })
()
override this.WriteJson(writer, value, serializer) =
let keyValueTypes = value.GetType().DictionaryKeyValueTypes().Single(); // Throws an exception if not exactly one.
let m = typeof<DictionaryConverter>.GetMethod("WriteJsonGeneric", BindingFlags.NonPublic ||| BindingFlags.Instance ||| BindingFlags.Public);
m.MakeGenericMethod(keyValueTypes).Invoke(this, [| writer; value; serializer |])
()
Which you would add to settings as follows:
let settings =
JsonSerializerSettings(
NullValueHandling = NullValueHandling.Ignore,
Converters = [| CompactUnionJsonConverter(true, true); DictionaryConverter() |]
)
And generates the following JSON for your bookData:
{
"Data": [
{
"Key": "Bid",
"Value": [
{
"Key": 1,
"Value": {
"S": 3.0,
"P": 5.0
}
}
]
},
{
"Key": "Ask",
"Value": []
}
]
}
Notes:
The converter works for all Dictionary<TKey, TValue> types (and subtypes).
The converter detects whether the dictionary keys will be serialized using a primitive contract, and if so, serializes the dictionary compactly as a JSON object. If not the dictionary is serialized as an array. You can observe this in the JSON shown above: the Dictionary<BookSide, BookSideData> dictionary is serialized as a JSON array, and the Dictionary<int, BookEntry> dictionary is serialized as a JSON object.
During deserialization the converter detects whether the incoming JSON value is an array or object, and adapts as required.
The converter is only implemented for the mutable .Net Dictionary<TKey, TValue> type. The logic would require some slight modification to deserialize the immutable Map<'Key,'Value> type.
Demo fiddle here.