fix: nullable enum converters - use WriteNullValue() and handle null token in Read

Nullable string enum Write now calls WriteNullValue() when HasValue is
false, instead of writing the string literal "null".

Nullable enum Read (all types) now checks reader.TokenType ==
JsonTokenType.Null and returns null before attempting to read the value.
Previously this check was only applied to non-string enum types.

Co-authored-by: alexaka1 <22166651+alexaka1@users.noreply.github.com>

Author: Copilot

modules/openapi-generator/src/main/resources/csharp/modelEnum.mustache

@@ -234,11 +234,9 @@
         /// <returns></returns>
         public override {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
-            {{^isString}}
             if (reader.TokenType == JsonTokenType.Null)
                 return null;
 
-            {{/isString}}
             {{#isInteger}}
             string rawValue = reader.{{>EnumJsonReaderMethod}}().ToString();
             {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}? result = {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}ValueConverter.FromStringOrDefault(rawValue);
@@ -296,15 +294,15 @@
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}? {{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}, JsonSerializerOptions options)
         {
-            {{^isString}}
             if ({{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}.HasValue)
+            {{^isString}}
                 writer.WriteNumberValue({{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}ValueConverter.ToJsonValue({{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}.Value));
-            else
-                writer.WriteNullValue();
             {{/isString}}
             {{#isString}}
-            writer.WriteStringValue({{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}.HasValue ? {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}ValueConverter.ToJsonValue({{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}.Value).ToString() : "null");
+                writer.WriteStringValue({{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}ValueConverter.ToJsonValue({{#lambda.camelcase_sanitize_param}}{{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{/lambda.camelcase_sanitize_param}}.Value).ToString());
             {{/isString}}
+            else
+                writer.WriteNullValue();
         }
     }
     {{/useGenericHost}}

samples/client/petstore/csharp/generichost/latest/ComposedEnum/src/Org.OpenAPITools/Model/AreaCode.cs

@@ -177,6 +177,9 @@ public class AreaCodeNullableJsonConverter : JsonConverter<AreaCode?>
         /// <returns></returns>
         public override AreaCode? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string? rawValue = reader.GetString();
 
             AreaCode? result = rawValue == null
@@ -197,7 +200,10 @@ public class AreaCodeNullableJsonConverter : JsonConverter<AreaCode?>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, AreaCode? areaCode, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(areaCode.HasValue ? AreaCodeValueConverter.ToJsonValue(areaCode.Value).ToString() : "null");
+            if (areaCode.HasValue)
+                writer.WriteStringValue(AreaCodeValueConverter.ToJsonValue(areaCode.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/latest/ComposedEnum/src/Org.OpenAPITools/Model/MarineAreaCode.cs

@@ -149,6 +149,9 @@ public class MarineAreaCodeNullableJsonConverter : JsonConverter<MarineAreaCode?
         /// <returns></returns>
         public override MarineAreaCode? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string? rawValue = reader.GetString();
 
             MarineAreaCode? result = rawValue == null
@@ -169,7 +172,10 @@ public class MarineAreaCodeNullableJsonConverter : JsonConverter<MarineAreaCode?
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, MarineAreaCode? marineAreaCode, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(marineAreaCode.HasValue ? MarineAreaCodeValueConverter.ToJsonValue(marineAreaCode.Value).ToString() : "null");
+            if (marineAreaCode.HasValue)
+                writer.WriteStringValue(MarineAreaCodeValueConverter.ToJsonValue(marineAreaCode.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/latest/ComposedEnum/src/Org.OpenAPITools/Model/StateTerritoryCode.cs

@@ -149,6 +149,9 @@ public class StateTerritoryCodeNullableJsonConverter : JsonConverter<StateTerrit
         /// <returns></returns>
         public override StateTerritoryCode? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string? rawValue = reader.GetString();
 
             StateTerritoryCode? result = rawValue == null
@@ -169,7 +172,10 @@ public class StateTerritoryCodeNullableJsonConverter : JsonConverter<StateTerrit
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, StateTerritoryCode? stateTerritoryCode, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(stateTerritoryCode.HasValue ? StateTerritoryCodeValueConverter.ToJsonValue(stateTerritoryCode.Value).ToString() : "null");
+            if (stateTerritoryCode.HasValue)
+                writer.WriteStringValue(StateTerritoryCodeValueConverter.ToJsonValue(stateTerritoryCode.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/latest/InlineEnumAnyOf/src/Org.OpenAPITools/Model/IconsSizeParameterAnyOf.cs

@@ -163,6 +163,9 @@ public class IconsSizeParameterAnyOfNullableJsonConverter : JsonConverter<IconsS
         /// <returns></returns>
         public override IconsSizeParameterAnyOf? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string? rawValue = reader.GetString();
 
             IconsSizeParameterAnyOf? result = rawValue == null
@@ -183,7 +186,10 @@ public class IconsSizeParameterAnyOfNullableJsonConverter : JsonConverter<IconsS
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, IconsSizeParameterAnyOf? iconsSizeParameterAnyOf, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(iconsSizeParameterAnyOf.HasValue ? IconsSizeParameterAnyOfValueConverter.ToJsonValue(iconsSizeParameterAnyOf.Value).ToString() : "null");
+            if (iconsSizeParameterAnyOf.HasValue)
+                writer.WriteStringValue(IconsSizeParameterAnyOfValueConverter.ToJsonValue(iconsSizeParameterAnyOf.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/net10/FormModels/src/Org.OpenAPITools/Model/ChildCatAllOfPetType.cs

@@ -134,6 +134,9 @@ public class ChildCatAllOfPetTypeNullableJsonConverter : JsonConverter<ChildCatA
         /// <returns></returns>
         public override ChildCatAllOfPetType? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string rawValue = reader.GetString();
 
             ChildCatAllOfPetType? result = rawValue == null
@@ -154,7 +157,10 @@ public class ChildCatAllOfPetTypeNullableJsonConverter : JsonConverter<ChildCatA
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, ChildCatAllOfPetType? childCatAllOfPetType, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(childCatAllOfPetType.HasValue ? ChildCatAllOfPetTypeValueConverter.ToJsonValue(childCatAllOfPetType.Value).ToString() : "null");
+            if (childCatAllOfPetType.HasValue)
+                writer.WriteStringValue(ChildCatAllOfPetTypeValueConverter.ToJsonValue(childCatAllOfPetType.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/net10/FormModels/src/Org.OpenAPITools/Model/CopyActivityAllOfSchema.cs

@@ -134,6 +134,9 @@ public class CopyActivityAllOfSchemaNullableJsonConverter : JsonConverter<CopyAc
         /// <returns></returns>
         public override CopyActivityAllOfSchema? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string rawValue = reader.GetString();
 
             CopyActivityAllOfSchema? result = rawValue == null
@@ -154,7 +157,10 @@ public class CopyActivityAllOfSchemaNullableJsonConverter : JsonConverter<CopyAc
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, CopyActivityAllOfSchema? copyActivityAllOfSchema, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(copyActivityAllOfSchema.HasValue ? CopyActivityAllOfSchemaValueConverter.ToJsonValue(copyActivityAllOfSchema.Value).ToString() : "null");
+            if (copyActivityAllOfSchema.HasValue)
+                writer.WriteStringValue(CopyActivityAllOfSchemaValueConverter.ToJsonValue(copyActivityAllOfSchema.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/net10/FormModels/src/Org.OpenAPITools/Model/EnumArraysArrayEnumInner.cs

@@ -148,6 +148,9 @@ public class EnumArraysArrayEnumInnerNullableJsonConverter : JsonConverter<EnumA
         /// <returns></returns>
         public override EnumArraysArrayEnumInner? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string rawValue = reader.GetString();
 
             EnumArraysArrayEnumInner? result = rawValue == null
@@ -168,7 +171,10 @@ public class EnumArraysArrayEnumInnerNullableJsonConverter : JsonConverter<EnumA
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, EnumArraysArrayEnumInner? enumArraysArrayEnumInner, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(enumArraysArrayEnumInner.HasValue ? EnumArraysArrayEnumInnerValueConverter.ToJsonValue(enumArraysArrayEnumInner.Value).ToString() : "null");
+            if (enumArraysArrayEnumInner.HasValue)
+                writer.WriteStringValue(EnumArraysArrayEnumInnerValueConverter.ToJsonValue(enumArraysArrayEnumInner.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/net10/FormModels/src/Org.OpenAPITools/Model/EnumArraysJustSymbol.cs

@@ -148,6 +148,9 @@ public class EnumArraysJustSymbolNullableJsonConverter : JsonConverter<EnumArray
         /// <returns></returns>
         public override EnumArraysJustSymbol? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string rawValue = reader.GetString();
 
             EnumArraysJustSymbol? result = rawValue == null
@@ -168,7 +171,10 @@ public class EnumArraysJustSymbolNullableJsonConverter : JsonConverter<EnumArray
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, EnumArraysJustSymbol? enumArraysJustSymbol, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(enumArraysJustSymbol.HasValue ? EnumArraysJustSymbolValueConverter.ToJsonValue(enumArraysJustSymbol.Value).ToString() : "null");
+            if (enumArraysJustSymbol.HasValue)
+                writer.WriteStringValue(EnumArraysJustSymbolValueConverter.ToJsonValue(enumArraysJustSymbol.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }

samples/client/petstore/csharp/generichost/net10/FormModels/src/Org.OpenAPITools/Model/EnumClass.cs

@@ -162,6 +162,9 @@ public class EnumClassNullableJsonConverter : JsonConverter<EnumClass?>
         /// <returns></returns>
         public override EnumClass? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
+            if (reader.TokenType == JsonTokenType.Null)
+                return null;
+
             string rawValue = reader.GetString();
 
             EnumClass? result = rawValue == null
@@ -182,7 +185,10 @@ public class EnumClassNullableJsonConverter : JsonConverter<EnumClass?>
         /// <param name="options"></param>
         public override void Write(Utf8JsonWriter writer, EnumClass? enumClass, JsonSerializerOptions options)
         {
-            writer.WriteStringValue(enumClass.HasValue ? EnumClassValueConverter.ToJsonValue(enumClass.Value).ToString() : "null");
+            if (enumClass.HasValue)
+                writer.WriteStringValue(EnumClassValueConverter.ToJsonValue(enumClass.Value).ToString());
+            else
+                writer.WriteNullValue();
         }
     }
 }