fix/additional properties to v3

CHANGELOG.md

@@ -2,15 +2,13 @@
 
 ## [3.0.1](https://github.com/microsoft/OpenAPI.NET/compare/v3.0.0...v3.0.1) (2025-11-17)
 
-
 ### Bug Fixes
 
 * empty strings should be quoted in yaml ([8d215f9](https://github.com/microsoft/OpenAPI.NET/commit/8d215f9ea8a780b1e2e8dd6cefb8d470cc35682d))
 * empty strings should be quoted in yaml ([0ca10db](https://github.com/microsoft/OpenAPI.NET/commit/0ca10db3bb9ffa937dd35862068926f3586d6991))
 
 ## [3.0.0](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.9...v3.0.0) (2025-11-11)
 
-
 ### ⚠ BREAKING CHANGES
 
 * adds support for OpenAPI 3.2.0
@@ -23,8 +21,12 @@
 
 * adds support for OpenAPI 3.2.0 ([765a8dd](https://github.com/microsoft/OpenAPI.NET/commit/765a8dd4d6efd1a31b6a76d282ccffa5877a845a))
 
-## [2.3.9](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.8...v2.3.9) (2025-11-06)
+## [2.3.10](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.9...v2.3.10) (2025-11-17)
 
+* empty strings should be quoted in yaml ([e919b33](https://github.com/microsoft/OpenAPI.NET/commit/e919b33e9d09159217066248483ef4c767865c82))
+* empty strings should be quoted in yaml ([0ca10db](https://github.com/microsoft/OpenAPI.NET/commit/0ca10db3bb9ffa937dd35862068926f3586d6991))
+
+## [2.3.9](https://github.com/microsoft/OpenAPI.NET/compare/v2.3.8...v2.3.9) (2025-11-06)
 
 ### Bug Fixes
 

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -471,17 +471,20 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalMap(OpenApiConstants.Properties, Properties, callback);
 
             // additionalProperties
-            if (AdditionalPropertiesAllowed)
+            if (AdditionalProperties is not null && version >= OpenApiSpecVersion.OpenApi3_0)
             {
                 writer.WriteOptionalObject(
                     OpenApiConstants.AdditionalProperties,
                     AdditionalProperties,
                     callback);
             }
-            else
+            // true is the default in earlier versions 3, no need to write it out
+            // boolean value is only supported for version 3 and earlier (version 2 is implemented in the other serialize method, the condition is a failsafe)
+            else if (!AdditionalPropertiesAllowed && version <= OpenApiSpecVersion.OpenApi3_0)
             {
                 writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);
             }
+            // not having anything is the same as having it set to true (v2/v3) or an empty schema (v3.1+)
 
             // description
             writer.WriteProperty(OpenApiConstants.Description, Description);
@@ -732,14 +735,9 @@ private void SerializeAsV2(
             });
 
             // additionalProperties
-            if (AdditionalPropertiesAllowed)
-            {
-                writer.WriteOptionalObject(
-                    OpenApiConstants.AdditionalProperties,
-                    AdditionalProperties,
-                    (w, s) => s.SerializeAsV2(w));
-            }
-            else
+            // a schema cannot be serialized in v2
+            // true is the default, no need to write it out
+            if (!AdditionalPropertiesAllowed)
             {
                 writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);
             }

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

@@ -684,6 +684,114 @@ public async Task SerializeConstAsEnumV20()
             Assert.False(v2Node.AsObject().ContainsKey("const"));
         }
 
+        [Fact]
+        public async Task SerializeAdditionalPropertiesAsV2DoesNotEmit()
+        {
+            var expected = @"{ }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalProperties = new OpenApiSchema()
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Fact]
+        public async Task SerializeAdditionalPropertiesAllowedAsV2DefaultDoesNotEmit()
+        {
+            var expected = @"{ }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalPropertiesAllowed = true
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Fact]
+        public async Task SerializeAdditionalPropertiesAllowedAsV2FalseEmits()
+        {
+            var expected = @"{ ""additionalProperties"": false }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalPropertiesAllowed = false
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2)]
+        public async Task SerializeAdditionalPropertiesAllowedAsV3PlusDefaultDoesNotEmit(OpenApiSpecVersion version)
+        {
+            var expected = @"{ }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalPropertiesAllowed = true
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(version);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Fact]
+        public async Task SerializeAdditionalPropertiesAllowedAsV3FalseEmits()
+        {
+            var expected = @"{ ""additionalProperties"": false }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalPropertiesAllowed = false
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2)]
+        public async Task SerializeAdditionalPropertiesAsV3PlusEmits(OpenApiSpecVersion version)
+        {
+            var expected = @"{ ""additionalProperties"": { } }";
+            // Given
+            var schema = new OpenApiSchema
+            {
+                AdditionalProperties = new OpenApiSchema()
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(version);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
 
         internal class SchemaVisitor : OpenApiVisitorBase
         {