fix(#204): extraneous field name in enum-like serialization

Author: Tarmil

src/FSharp.SystemTextJson/Union.fs

@@ -297,6 +297,12 @@ module private Case =
         | JsonName.Int name -> writer.WriteNumber(fsOptions.UnionTagName, name)
         | JsonName.Bool name -> writer.WriteBoolean(fsOptions.UnionTagName, name)
 
+    let writeCaseNameAsValue (writer: Utf8JsonWriter) (case: Case) =
+        match case.Names[0] with
+        | JsonName.String name -> writer.WriteStringValue(name)
+        | JsonName.Int name -> writer.WriteNumberValue(name)
+        | JsonName.Bool name -> writer.WriteBooleanValue(name)
+
 
 type JsonUnionConverter<'T>
     internal (options: JsonSerializerOptions, fsOptions: JsonFSharpOptionsRecord, cases: UnionCaseInfo[]) =
@@ -614,12 +620,6 @@ type JsonUnionConverter<'T>
         else
             writeFieldsAsArray writer case value options
 
-    let writeCaseNameAsValue (writer: Utf8JsonWriter) (case: Case) =
-        match case.Names[0] with
-        | JsonName.String name -> writer.WriteStringValue(name)
-        | JsonName.Int name -> writer.WriteNumberValue(name)
-        | JsonName.Bool name -> writer.WriteBooleanValue(name)
-
     let writeAdjacentTag (writer: Utf8JsonWriter) (case: Case) (value: obj) (options: JsonSerializerOptions) =
         writer.WriteStartObject()
         Case.writeCaseNameAsField fsOptions writer case
@@ -641,7 +641,7 @@ type JsonUnionConverter<'T>
             writeFieldsAsRestOfObject writer case value options
         else
             writer.WriteStartArray()
-            writeCaseNameAsValue writer case
+            Case.writeCaseNameAsValue writer case
             writeFieldsAsRestOfArray writer case value options
 
     let writeUntagged (writer: Utf8JsonWriter) (case: Case) (value: obj) (options: JsonSerializerOptions) =
@@ -680,7 +680,7 @@ type JsonUnionConverter<'T>
             let tag = tagReader value
             let case = cases[tag]
             if unwrapFieldlessTags && case.Fields.Length = 0 then
-                writeCaseNameAsValue writer case
+                Case.writeCaseNameAsValue writer case
             else
                 match baseFormat with
                 | JsonUnionEncoding.AdjacentTag -> writeAdjacentTag writer case value options
@@ -792,7 +792,7 @@ type JsonEnumLikeUnionConverter<'T> internal (options: JsonSerializerOptions, fs
 
     override this.Write(writer, value, _options) =
         let tag = tagReader value
-        Case.writeCaseNameAsField fsOptions writer cases[tag]
+        Case.writeCaseNameAsValue writer cases[tag]
 
     override this.WriteAsPropertyName(writer, value, _options) =
         let tag = tagReader value

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

@@ -126,3 +126,20 @@ let ``regression #172`` () =
     options.Converters.Add(JsonFSharpConverter())
     Assert.Equal("{\"X\":null}", JsonSerializer.Serialize(x, options))
     Assert.Equal("{\"Y\":null}", JsonSerializer.Serialize(y, options))
+
+module ``Regression #204`` =
+    type Enum =
+        | Case1
+        | Case2
+
+    type Record = { Field: Enum }
+
+    let options =
+        JsonFSharpOptions.Default().WithUnionUnwrapFieldlessTags().ToJsonSerializerOptions()
+
+    let actual () =
+        JsonSerializer.Serialize({ Field = Case2 }, options)
+
+[<Fact>]
+let ``regression #204`` () =
+    Assert.Equal("""{"Field":"Case2"}""", ``Regression #204``.actual ())