How Can I Deserialize Json With F#?

Question

I am attempting to deserialize some very simple json into F# with Newtonsoft.Json 5.0.4:

#if INTERACTIVE
#r "C:/Users/OCatenacci/fshacks/CreateWeeklySWEEventsEmail/packages/Newtonsoft.Json.5.0.4/lib/net40/Newtonsoft.Json.dll"
#endif 

open System
open Newtonsoft.Json

type meta() = class
    member val count = 0 with get, set
    end

let testjson = """{
    "meta": {
        "count": 15
    }
}"""


let o = JsonConvert.DeserializeObject<meta>(testjson)

meta always gets a 0 in the count. By the way, I was originally defining meta like this:

type meta = {
   count: int
}

And I changed to using the Automatic property because I thought that Newtonsoft.Json might not be able to construct the object correctly.

I would be surprised if my version of F#/Windows makes a difference in this case but just for sake of completeness: I'm trying this with the F# 3.0 Repl (11.0.60315.1) and I'm running on a Win7 x64 (SP1) box.

Answer 1

Try removing the outer-most set of curly braces from your JSON. Right now, you have an object containing an instance of meta. That's probably throwing off JSON.NET. In other words, I got it to run fine by using:

let testjson = "{ 'count' : 15 }"

[Updated base on comments]

Alternately, you could keep the JSON as-is, and provide a more complex type tree in F#. For example:

type Foo =
  { meta : Meta }
and Meta =
  { count : int }

Answer 2

With FSharp.Json library you can use just record types - no need to define classes anymore. Structure of records provided above is just enough:

type Foo =
  { meta : Meta }
and Meta =
  { count : int }

let testjson = """{"meta": {"count": 15}}"""

open FSharp.Json
let data = Json.deserialize<Foo> testjson

Disclosure: I'm author of FSharp.Json library.