Merge pull request #2853 from microsoft/copilot/fix-openapi-deserializer-nullable-issue

fix(reader): preserve Null flag when nullable appears before type in V3.0/V3.1/V3.2 deserializers

Author: baywet

src/Microsoft.OpenApi/Reader/V31/OpenApiSchemaDeserializer.cs

@@ -198,14 +198,17 @@ internal static partial class OpenApiV31Deserializer
             "type",
             (o, n, doc) => 
             {
+                // Preserve any Null flag set by a preceding "nullable: true" handler
+                var preserveNull = o.Type.HasValue && o.Type.Value.HasFlag(JsonSchemaType.Null);
                 if (n is ValueNode)
                 {
-                    o.Type = n.GetScalarValue()?.ToJsonSchemaType();
+                    var parsedType = n.GetScalarValue()?.ToJsonSchemaType();
+                    o.Type = preserveNull ? parsedType | JsonSchemaType.Null : parsedType;
                 }
                 else
                 {
                     var list = n.CreateSimpleList((n2, p) => n2.GetScalarValue(), doc);
-                    JsonSchemaType combinedType = 0;
+                    JsonSchemaType combinedType = preserveNull ? JsonSchemaType.Null : 0;
                     foreach(var type in list.Where(static t => t is not null).Select(static t => t!.ToJsonSchemaType()))
                     {
                         combinedType |= type;

src/Microsoft.OpenApi/Reader/V32/OpenApiSchemaDeserializer.cs

@@ -198,14 +198,17 @@ internal static partial class OpenApiV32Deserializer
             "type",
             (o, n, doc) => 
             {
+                // Preserve any Null flag set by a preceding "nullable: true" handler
+                var preserveNull = o.Type.HasValue && o.Type.Value.HasFlag(JsonSchemaType.Null);
                 if (n is ValueNode)
                 {
-                    o.Type = n.GetScalarValue()?.ToJsonSchemaType();
+                    var parsedType = n.GetScalarValue()?.ToJsonSchemaType();
+                    o.Type = preserveNull ? parsedType | JsonSchemaType.Null : parsedType;
                 }
                 else
                 {
                     var list = n.CreateSimpleList((n2, p) => n2.GetScalarValue(), doc);
-                    JsonSchemaType combinedType = 0;
+                    JsonSchemaType combinedType = preserveNull ? JsonSchemaType.Null : 0;
                     foreach(var type in list.Where(static t => t is not null).Select(static t => t!.ToJsonSchemaType()))
                     {
                         combinedType |= type;

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiSchemaTests.cs

@@ -132,6 +132,18 @@ public void ParseSchemaWithTypeArrayWorks()
             Assert.Equivalent(expected, actual);
         }
 
+        [Theory]
+        [InlineData(@"{ ""nullable"": true, ""type"": ""string"" }")]
+        [InlineData(@"{ ""type"": ""string"", ""nullable"": true }")]
+        public void ParseSchemaWithNullableBeforeOrAfterTypePreservesNullFlag(string schemaJson)
+        {
+            // Act
+            var schema = OpenApiModelFactory.Parse<OpenApiSchema>(schemaJson, OpenApiSpecVersion.OpenApi3_1, new(), out _, "json", SettingsFixture.ReaderSettings);
+
+            // Assert
+            Assert.Equal(JsonSchemaType.String | JsonSchemaType.Null, schema.Type);
+        }
+
         [Fact]
         public void TestSchemaCopyConstructorWithTypeArrayWorks()
         {

test/Microsoft.OpenApi.Readers.Tests/V32Tests/OpenApiSchemaTests.cs

@@ -131,6 +131,18 @@ public void ParseSchemaWithTypeArrayWorks()
             Assert.Equivalent(expected, actual);
         }
 
+        [Theory]
+        [InlineData(@"{ ""nullable"": true, ""type"": ""string"" }")]
+        [InlineData(@"{ ""type"": ""string"", ""nullable"": true }")]
+        public void ParseSchemaWithNullableBeforeOrAfterTypePreservesNullFlag(string schemaJson)
+        {
+            // Act
+            var schema = OpenApiModelFactory.Parse<OpenApiSchema>(schemaJson, OpenApiSpecVersion.OpenApi3_2, new(), out _, "json", SettingsFixture.ReaderSettings);
+
+            // Assert
+            Assert.Equal(JsonSchemaType.String | JsonSchemaType.Null, schema.Type);
+        }
+
         [Fact]
         public void TestSchemaCopyConstructorWithTypeArrayWorks()
         {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiSchemaTests.cs

@@ -133,6 +133,18 @@ public void ParsePathFragmentShouldSucceed()
                 }, openApiAny);
         }
 
+        [Theory]
+        [InlineData(@"{ ""nullable"": true, ""type"": ""string"" }")]
+        [InlineData(@"{ ""type"": ""string"", ""nullable"": true }")]
+        public void ParseSchemaWithNullableBeforeOrAfterTypePreservesNullFlag(string schemaJson)
+        {
+            // Act
+            var schema = OpenApiModelFactory.Parse<OpenApiSchema>(schemaJson, OpenApiSpecVersion.OpenApi3_0, new(), out _, "json", SettingsFixture.ReaderSettings);
+
+            // Assert
+            Assert.Equal(JsonSchemaType.String | JsonSchemaType.Null, schema.Type);
+        }
+
         [Fact]
         public void ParseDictionarySchemaShouldSucceed()
         {