Deserializing F# Map by Json.Net - json

Question

I have one simply question: Is it possible to parse F# Map type from json? Because when I try it (With F# Map<string, string>), it is easy to serialize and it looks how it have to, but when I try to deserialize it is throwing an exception.
Newtonsoft.Json.JsonSerializationException: Unable to find a default constructor to use for type Microsoft.FSharp.Collections.FSharpMap`2[System.Int32,System.String]. Path '1', line 2, position 7.
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateNewDictionary (Newtonsoft.Json.JsonReader reader, Newtonsoft.Json.Serialization.JsonDictionaryContract contract, System.Boolean& createdFromNonDefaultConstructor) [0x00000] in <filename unknown>:0
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObject (Newtonsoft.Json.JsonReader reader, System.Type objectType, Newtonsoft.Json.Serialization.JsonContract contract, Newtonsoft.Json.Serialization.JsonProperty member, Newtonsoft.Json.Serialization.JsonContainerContract containerContract, Newtonsoft.Json.Serialization.JsonProperty containerMember, System.Object existingValue) [0x00000] in <filename unknown>:0
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal (Newtonsoft.Json.JsonReader reader, System.Type objectType, Newtonsoft.Json.Serialization.JsonContract contract, Newtonsoft.Json.Serialization.JsonProperty member, Newtonsoft.Json.Serialization.JsonContainerContract containerContract, Newtonsoft.Json.Serialization.JsonProperty containerMember, System.Object existingValue) [0x00000] in <filename unknown>:0
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize (Newtonsoft.Json.JsonReader reader, System.Type objectType, Boolean checkAdditionalContent) [0x00000] in <filename unknown>:0
And it is deserializing from classic:
Map.ofList [ ("1", "one"); ("2", "two"); ("3", "three") ]
The resulting JSON looks like C# dictionary
{
"1": "one",
"2": "two",
"3": "three"
}
It is serializing without settings (Only indentation). So is it possible to serialize this, or is there some working workaround?
Thanks for answer

Answer 1

You can make your own converter to do this. It's a lot of reflection and constructing appropriate generic types, but it can be done.
You first deserialize to a Dictionary<Key, Val>, then create and fill a List<Tuple<Key, Val>> manually via reflection (because the Map constructor requires Tuples, not KeyValuePairs), then finally pass that into the Map constructor.
Not sure if there's an easier way, but this is what I came up with:
open System
open System.Collections
open System.Collections.Generic
open Newtonsoft.Json
let mapConverter = {
new JsonConverter() with
override x.CanConvert(t:Type) =
t.IsGenericType && t.GetGenericTypeDefinition() = typedefof<Map<_, _>>
override x.WriteJson(writer, value, serializer) =
serializer.Serialize(writer, value)
override x.ReadJson(reader, t, _, serializer) =
let genArgs = t.GetGenericArguments()
let generify (t:Type) = t.MakeGenericType genArgs
let tupleType = generify typedefof<Tuple<_, _>>
let listType = typedefof<List<_>>.MakeGenericType tupleType
let create (t:Type) types = (t.GetConstructor types).Invoke
let list = create listType [||] [||] :?> IList
let kvpType = generify typedefof<KeyValuePair<_, _>>
for kvp in serializer.Deserialize(reader, generify typedefof<Dictionary<_, _>>) :?> IEnumerable do
let get name = (kvpType.GetProperty name).GetValue(kvp, null)
list.Add (create tupleType genArgs [|get "Key"; get "Value"|]) |> ignore
create (generify typedefof<Map<_, _>>) [|listType|] [|list|]
}
Once you have your converter, then you just pass it into the DeserializeObject method and JsonConvert will use it wherever appropriate.
let str = JsonConvert.SerializeObject (Map<_, _> [333, 1234])
JsonConvert.DeserializeObject<Map<int, int>>(str, mapConverter)
The nice thing about doing it this way is that if you've got a big/deep record where your Map is just a single field, then it'll work with that too--you don't have to go changing your record structure to use Dictionaries just to support serialization.

Answer 2

This functionality became part of JSON.Net in version 6.0.3. (April 30th, 2014)
But, if you are stuck for some reason using an earlier version then a simplified (and more efficient as less reflection) version of Dax Fohl's version could be:
type mapConvert<'f,'t when 'f : comparison>() =
static member readJson (reader:JsonReader, serializer:JsonSerializer) =
serializer.Deserialize<Dictionary<'f, 't>> (reader)
|> Seq.map (fun kv -> kv.Key, kv.Value)
|> Map.ofSeq
let mapConverter = {
new JsonConverter() with
override __.CanConvert (t:Type) =
t.IsGenericType && t.GetGenericTypeDefinition() = typedefof<Map<_, _>>
override __.WriteJson (writer, value, serializer) =
serializer.Serialize(writer, value)
override __.ReadJson (reader, t, _, serializer) =
let converter =
typedefof<mapConvert<_,_>>.MakeGenericType (t.GetGenericArguments())
let readJson =
converter.GetMethod("readJson")
readJson.Invoke(null, [| reader; serializer |])
}

Answer 3

The problem is that json.net can't construct a Map<int,string>. However if you deserialize to a regular .net Dictionary<int,string> it will work, as the json is the same.

Answer 4

You can't serialize F#'s Map directly, since it has no default contructor (constructor with no parameter) at all.
This is the original documentation of F# map: (from http://msdn.microsoft.com/en-us/library/ee353686%28v=vs.110%29.aspx)
[<Sealed>]
type Map<[<EqualityConditionalOnAttribute>] 'Key,[<ComparisonConditionalOnAttribute>] [<EqualityConditionalOnAttribute>] 'Value (requires comparison)> =
class
interface IEnumerable
interface IComparable
interface IEnumerable
interface ICollection
interface IDictionary
new Map : seq<'Key * 'Value> -> Map< 'Key, 'Value>
member this.Add : 'Key * 'Value -> Map<'Key, 'Value>
member this.ContainsKey : 'Key -> bool
member this.Remove : 'Key -> Map<'Key, 'Value>
member this.TryFind : 'Key -> 'Value option
member this.Count : int
member this.IsEmpty : bool
member this.Item ('Key) : 'Value
end
As you see above, Map doesn't have default constructor but the serializer need a class with default constructor.
The best way to serialize a map is mapping the map to be regular .NET dictionary, but then the new dictionary doesn't have all of the advantages of F#'s Map, especially the immutability of F#'s Map.