UnionConverter: Handle Nested Unions (#52)

Author: bartelink

CHANGELOG.md

@@ -13,6 +13,7 @@ The `Unreleased` section name is replaced by the expected version of next releas
 ### Removed
 ### Fixed
 
+- `UnionConverter`: Handle nested unions [#52](https://github.com/jet/FsCodec/pull/52)
 - `UnionConverter`: Support overriding discriminator without needing to nominate a `catchAllCase` [#51](https://github.com/jet/FsCodec/pull/51)
 
 <a name="2.1.0"></a>

src/FsCodec.NewtonsoftJson/UnionConverter.fs

@@ -31,7 +31,7 @@ module private Union =
 
     let getUnion = memoize createUnion
 
-    /// Paralells F# behavior wrt how it generates a DU's underlying .NET Type
+    /// Parallels F# behavior wrt how it generates a DU's underlying .NET Type
     let inline isInlinedIntoUnionItem (t : Type) =
         t = typeof<string>
         || t.IsValueType
@@ -41,6 +41,7 @@ module private Union =
                 || t.GetGenericTypeDefinition().IsValueType)) // Nullable<T>
 
     let typeHasJsonConverterAttribute = memoize (fun (t : Type) -> t.IsDefined(typeof<JsonConverterAttribute>))
+    let typeIsUnionWithConverterAttribute = memoize (fun (t : Type) -> isUnion t && typeHasJsonConverterAttribute t)
 
     let propTypeRequiresConstruction (propertyType : Type) =
         not (isInlinedIntoUnionItem propertyType)
@@ -91,7 +92,7 @@ type UnionConverter private (discriminator : string, ?catchAllCase) =
         writer.WriteValue(case.Name)
 
         match fieldInfos with
-        | [| fi |] ->
+        | [| fi |] when not (Union.typeIsUnionWithConverterAttribute fi.PropertyType) ->
             match fieldValues.[0] with
             | null when serializer.NullValueHandling = NullValueHandling.Ignore -> ()
             | fv ->

tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs

@@ -371,3 +371,118 @@ module ``Unmatched case handling`` =
                 && string jo.["case"]="CaseUnknown" @>
         let expected  = "{\r\n  \"case\": \"CaseUnknown\",\r\n  \"a\": \"s\",\r\n  \"b\": 1,\r\n  \"c\": true\r\n}".Replace("\r\n",Environment.NewLine)
         test <@ expected = string jo @>
+
+module Nested =
+
+    [<JsonConverter(typeof<UnionConverter>)>]
+    type U =
+        | B of NU
+        | C of UUA
+        | D of UU
+        | E of E
+        | EA of E[]
+        | R of {| a : int; b : NU |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter>)>]
+        NU =
+        | A of string
+        | B of int
+        | R of {| a : int; b : NU |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter>)>]
+        UU =
+        | A of string
+        | B of int
+        | E of E
+        | EO of E option
+        | R of {| a: int; b: string |}
+        | S
+    and [<JsonConverter(typeof<UnionConverter>, "case2")>]
+        UUA =
+        | A of string
+        | B of int
+        | E of E
+        | EO of E option
+        | R of {| a: int; b: string |}
+        | S
+    and [<JsonConverter(typeof<TypeSafeEnumConverter>)>]
+        E =
+        | V1
+        | V2
+
+    let [<FsCheck.Xunit.Property>] ``can nest`` (value : U) =
+        let ser = Serdes.Serialize value
+        test <@ value = Serdes.Deserialize ser @>
+
+    let [<Fact>] ``nesting Unions represents child as item`` () =
+        let v : U = U.C(UUA.B 42)
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"B","Item":42}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+    let [<Fact>] ``TypeSafeEnum converts direct`` () =
+        let v : U = U.C (UUA.E E.V1)
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"E","Item":"V1"}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.E E.V2
+        let ser = Serdes.Serialize v
+        """{"case":"E","Item":"V2"}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.EA [|E.V2; E.V2|]
+        let ser = Serdes.Serialize v
+        """{"case":"EA","Item":["V2","V2"]}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C (UUA.EO (Some E.V1))
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"EO","Item":"V1"}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C (UUA.EO None)
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"EO","Item":null}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+        let v : U = U.C UUA.S
+        let ser = Serdes.Serialize v
+        """{"case":"C","Item":{"case2":"S"}}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+/// And for everything else, JsonIsomorphism allows plenty ways of customizing the encoding and/or decoding
+module IsomorphismUnionEncoder =
+
+    type [<JsonConverter(typeof<TopConverter>)>]
+        Top =
+        | S
+        | N of Nested
+    and Nested =
+        | A
+        | B of int
+    and TopConverter() =
+        inherit JsonIsomorphism<Top, Flat<int>>()
+        override __.Pickle value =
+            match value with
+            | S -> { disc = TS; v = None }
+            | N A -> { disc = TA; v = None }
+            | N (B v) -> { disc = TB; v = Some v }
+        override __.UnPickle flat =
+            match flat with
+            | { disc = TS } -> S
+            | { disc = TA } -> N A
+            | { disc = TB; v = v} -> N (B (Option.get v))
+    and Flat<'T> = { disc : JiType; v : 'T option }
+    and [<JsonConverter(typeof<TypeSafeEnumConverter>)>]
+        JiType = TS | TA | TB
+
+    let [<Fact>] ``Can control the encoding to the nth degree`` () =
+        let v : Top = N (B 42)
+        let ser = Serdes.Serialize v
+        """{"disc":"TB","v":42}""" =! ser
+        test <@ v = Serdes.Deserialize ser @>
+
+    let [<FsCheck.Xunit.Property>] ``can roundtrip`` (value : Top) =
+        let ser = Serdes.Serialize value
+        test <@ value = Serdes.Deserialize ser @>