Add Support for Untagged Unwrapped Single Cases

Author: daniellittledev

src/FSharp.SystemTextJson/Union.fs

@@ -210,6 +210,41 @@ type JsonUnionConverter<'T>
         else
             ValueNone
 
+    let casesByJsonType =
+        if fsOptions.UnionEncoding.HasFlag JsonUnionEncoding.Untagged && fsOptions.UnionEncoding.HasFlag JsonUnionEncoding.UnwrapSingleFieldCases then
+            let dict = Dictionary<JsonTokenType, Case>()
+            for c in cases do
+                let clrType = c.Fields[0].Type
+                let typeCode = Type.GetTypeCode(c.Fields[0].Type)
+                match typeCode with
+                | TypeCode.Byte
+                | TypeCode.SByte
+                | TypeCode.UInt16
+                | TypeCode.UInt32
+                | TypeCode.UInt64
+                | TypeCode.Int16
+                | TypeCode.Int32
+                | TypeCode.Int64
+                | TypeCode.Decimal
+                | TypeCode.Double
+                | TypeCode.Single ->
+                    dict[JsonTokenType.Number] <- c
+                | TypeCode.Boolean ->
+                    dict[JsonTokenType.True] <- c
+                    dict[JsonTokenType.False] <- c
+                | TypeCode.DateTime
+                | TypeCode.String ->
+                    dict[JsonTokenType.String] <- c
+                | TypeCode.Object when typeof<System.Collections.IEnumerable>.IsAssignableFrom(clrType) ->
+                    dict[JsonTokenType.StartArray] <- c
+                | TypeCode.Object ->
+                    dict[JsonTokenType.StartObject] <- c
+                | _ -> 
+                    ()
+            ValueSome dict
+        else
+            ValueNone
+
     let getJsonName (reader: byref<Utf8JsonReader>) =
         match reader.TokenType with
         | JsonTokenType.True -> JsonName.Bool true
@@ -533,6 +568,24 @@ type JsonUnionConverter<'T>
             | ValueNone -> failExpecting "case field" &reader ty
         | _ -> failExpecting "case field" &reader ty
 
+    let getCaseByElementType (reader: byref<Utf8JsonReader>) =
+        let found =
+            match casesByJsonType with
+            | ValueNone ->
+                ValueNone
+            | ValueSome d ->
+                match d.TryGetValue(reader.TokenType) with
+                | true, p -> ValueSome p
+                | false, _ -> ValueNone
+        match found with
+        | ValueNone -> failf "Unknown case for union type %s due to unmatched field type: %s" ty.FullName (reader.GetString())
+        | ValueSome case -> case
+
+    let readUnwrapedUntagged (reader: byref<Utf8JsonReader>) =
+        let case = getCaseByElementType &reader
+        let field = JsonSerializer.Deserialize(&reader, case.Fields[0].Type, options)
+        case.Ctor [| field |] :?> 'T
+
     let writeFieldsAsRestOfArray (writer: Utf8JsonWriter) (case: Case) (value: obj) (options: JsonSerializerOptions) =
         let fields = case.Fields
         let values = case.Dector value
@@ -614,7 +667,10 @@ type JsonUnionConverter<'T>
             writeFieldsAsRestOfArray writer case value options
 
     let writeUntagged (writer: Utf8JsonWriter) (case: Case) (value: obj) (options: JsonSerializerOptions) =
-        writeFieldsAsObject writer case value options
+        if case.UnwrappedSingleField then
+            JsonSerializer.Serialize(writer, (case.Dector value)[0], case.Fields[0].Type, options)
+        else
+            writeFieldsAsObject writer case value options
 
     override _.Read(reader, _typeToConvert, options) =
         match reader.TokenType with
@@ -633,11 +689,14 @@ type JsonUnionConverter<'T>
             | JsonUnionEncoding.ExternalTag -> readExternalTag &reader options
             | JsonUnionEncoding.InternalTag -> readInternalTag &reader options
             | UntaggedBit ->
-                if not hasDistinctFieldNames then
+                if fsOptions.UnionEncoding.HasFlag JsonUnionEncoding.UnwrapSingleFieldCases then
+                    readUnwrapedUntagged &reader
+                elif not hasDistinctFieldNames then
                     failf
                         "Union %s can't be deserialized as Untagged because it has duplicate field names across unions"
                         ty.FullName
-                readUntagged &reader options
+                else
+                    readUntagged &reader options
             | _ -> failf "Invalid union encoding: %A" fsOptions.UnionEncoding
 
     override _.Write(writer, value, options) =

tests/FSharp.SystemTextJson.Tests/Test.Union.fs

@@ -2017,6 +2017,40 @@ module Struct =
             JsonSerializer.Serialize(Bc("test", true), unwrapSingleFieldCasesOptions)
         )
 
+    let untaggedUnwrappedSingleFieldCasesOptions = JsonSerializerOptions()
+
+    untaggedUnwrappedSingleFieldCasesOptions.Converters.Add(
+        JsonFSharpConverter(JsonUnionEncoding.Untagged ||| JsonUnionEncoding.UnwrapSingleFieldCases)
+    )
+
+    type Object = { name: string }
+    type ChoiceOf5 = Choice<int, string, bool, int list, Object>
+    [<Fact>]
+    let ``serialize untagged unwrapped single-field cases`` () =
+        Assert.Equal("1", JsonSerializer.Serialize(Choice1Of5 1, untaggedUnwrappedSingleFieldCasesOptions))
+        Assert.Equal("\"F#\"", JsonSerializer.Serialize(Choice2Of5 "F#", untaggedUnwrappedSingleFieldCasesOptions))
+        Assert.Equal("false", JsonSerializer.Serialize(Choice3Of5 false, untaggedUnwrappedSingleFieldCasesOptions))
+        Assert.Equal("[1,2]", JsonSerializer.Serialize(Choice4Of5 [1;2], untaggedUnwrappedSingleFieldCasesOptions))
+        Assert.Equal("{name:\"Object\"}", JsonSerializer.Serialize(Choice5Of5 { name = "Object" }, untaggedUnwrappedSingleFieldCasesOptions))
+
+    [<Fact>]
+    let ``deserialize untagged unwrapped single-field cases`` () =
+        let choice1 = JsonSerializer.Deserialize<ChoiceOf5>("1", untaggedUnwrappedSingleFieldCasesOptions)
+        Assert.Equal(Choice1Of5 1, choice1)
+
+        let choice2 = JsonSerializer.Deserialize<ChoiceOf5>("\"F#\"", untaggedUnwrappedSingleFieldCasesOptions)
+        let expected2: ChoiceOf5 = Choice2Of5 "F#"
+        Assert.Equal(expected2, choice2)
+
+        let choice3 = JsonSerializer.Deserialize<ChoiceOf5>("false", untaggedUnwrappedSingleFieldCasesOptions)
+        Assert.Equal(Choice3Of5 false, choice3)
+        
+        let choice4 = JsonSerializer.Deserialize<ChoiceOf5>("[1,2]", untaggedUnwrappedSingleFieldCasesOptions)
+        Assert.Equal(Choice4Of5 [1;2], choice4)
+
+        let choice5 = JsonSerializer.Deserialize<ChoiceOf5>("""{"name":"Object"}""", untaggedUnwrappedSingleFieldCasesOptions)
+        Assert.Equal(Choice5Of5 { name = "Object" }, choice5)
+
     let unwrapFieldlessTagsOptions = JsonSerializerOptions()
     unwrapFieldlessTagsOptions.Converters.Add(JsonFSharpConverter(JsonUnionEncoding.UnwrapFieldlessTags))