Better nullability round-tripping

Author: Youssef1313

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -550,7 +550,7 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             }
 
             // type
-            var serializedTypeProperty = TrySerializeTypeProperty(writer, version);
+            SerializeTypeProperty(writer, version);
 
             // allOf
             writer.WriteOptionalCollection(OpenApiConstants.AllOf, AllOf, callback);
@@ -595,13 +595,13 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalObject(OpenApiConstants.Default, Default, (w, d) => w.WriteAny(d));
 
             // nullable
-            if (version == OpenApiSpecVersion.OpenApi3_0 && serializedTypeProperty)
+            if (version == OpenApiSpecVersion.OpenApi3_0)
             {
                 // https://spec.openapis.org/oas/v3.0.4.html#fixed-fields-20
                 // This keyword only takes effect if type is explicitly defined within the same Schema Object.
                 //
-                // If the user explicitly set IsNullable to true, we serialize it even if redundant.
-                // But if **we** are inferring it (from oneOf/anyOf), we don't serialize it when it's redundant.
+                // We don't care to avoid an unnecessary serialization.
+                // So, we attempt to serialize it regardless of whether or not a type property was serialized.
                 SerializeNullable(writer, version);
             }
 
@@ -838,7 +838,7 @@ private void SerializeAsV2(
             writer.WriteStartObject();
 
             // type
-            TrySerializeTypeProperty(writer, OpenApiSpecVersion.OpenApi2_0);
+            SerializeTypeProperty(writer, OpenApiSpecVersion.OpenApi2_0);
 
             // description
             writer.WriteProperty(OpenApiConstants.Description, Description);
@@ -1006,14 +1006,13 @@ private void SerializeAsV2(
             writer.WriteEndObject();
         }
 
-        private bool TrySerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion version, JsonSchemaType? inferredType = null)
+        private void SerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion version)
         {
-            // Use original type or inferred type when the explicit type is not set
-            var typeToUse = Type ?? inferredType;
+            var typeToUse = Type;
 
             if (typeToUse is null)
             {
-                return false;
+                return;
             }
 
             switch (version)
@@ -1023,15 +1022,15 @@ private bool TrySerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion
                     if (typeWithoutNull != 0 && !HasMultipleTypes(typeWithoutNull))
                     {
                         writer.WriteProperty(OpenApiConstants.Type, typeWithoutNull.ToFirstIdentifier());
-                        return true;
+                        return;
                     }
                     break;
                 default:
                     WriteUnifiedSchemaType(typeToUse.Value, writer);
-                    return true;
+                    return;
             }
 
-            return false;
+            return;
         }
 
         private JsonNode? GetCompatibilityExample()

src/Microsoft.OpenApi/Reader/JsonNodeHelper.cs

@@ -42,7 +42,13 @@ public static List<JsonNode> CreateListOfAny(this JsonNode? node, ParsingContext
                 throw new OpenApiReaderException("Cannot create a list from this type of node.", context);
             }
 
-            return jsonArray.OfType<JsonNode>().ToList();
+            var list = new List<JsonNode>(jsonArray.Count);
+            foreach (var element in jsonArray)
+            {
+                list.Add(element ?? JsonNullSentinel.JsonNull);
+            }
+
+            return list;
         }
 
         public static List<T> CreateSimpleList<T>(this JsonNode? node, Func<JsonNode, OpenApiDocument?, T> map, OpenApiDocument? openApiDocument, ParsingContext context)

src/Microsoft.OpenApi/Reader/V3/OpenApiSchemaDeserializer.cs

@@ -413,6 +413,13 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
                 }
             }
 
+            if (schema.Type is null && schema.Enum is { Count: 1 } &&
+                schema.Enum[0].IsJsonNullSentinel())
+            {
+                schema.Enum = null;
+                schema.Type = JsonSchemaType.Null;
+            }
+
             return schema;
         }
     }

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -965,7 +965,8 @@ public async Task SerializeOneOfWithNullAsV3ShouldUseNullableAsync()
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "maxLength": 10,
@@ -1009,7 +1010,8 @@ public async Task SerializeOneOfWithNullAndMultipleSchemasAsV3ShouldMarkItAsNull
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "type": "string"
@@ -1061,7 +1063,8 @@ public async Task SerializeAnyOfWithNullAsV3ShouldUseNullableAsync()
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "type": "object",
@@ -1108,7 +1111,8 @@ public async Task SerializeAnyOfWithNullAndMultipleSchemasAsV3ShouldApplyNullabl
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "minLength": 1,
@@ -1153,7 +1157,8 @@ public async Task SerializeOneOfWithOnlyNullAsV3ShouldJustBeNullableAsync()
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     }
                   ]
                 }
@@ -1256,7 +1261,8 @@ public async Task SerializeOneOfWithNullAndRefAsV3ShouldUseNullableAsync()
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "$ref": "#/components/schemas/Pet"
@@ -2050,7 +2056,8 @@ public async Task SerializeNullableEnumWith3_0()
                     {
                       "enum": [
                         null
-                      ]
+                      ],
+                      "nullable": true
                     },
                     {
                       "enum": [
@@ -2099,7 +2106,8 @@ public async Task SerializeNullableTypeWith3_0()
                 {
                   "enum": [
                     null
-                  ]
+                  ],
+                  "nullable": true
                 }
                 """;