Do not emit type:object when schema type is only nullable

Author: bkoelman

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -765,15 +765,13 @@ Extensions is not null &&
             }
             else if (!HasMultipleTypes(type.Value))
             {
-
                 switch (version)
                 {
                     case OpenApiSpecVersion.OpenApi3_1 when isNullable:
                         UpCastSchemaTypeToV31(type.Value, writer);
                         break;
                     case OpenApiSpecVersion.OpenApi3_0 when isNullable && type.Value == JsonSchemaType.Null:
                         writer.WriteProperty(OpenApiConstants.Nullable, true);
-                        writer.WriteProperty(OpenApiConstants.Type, JsonSchemaType.Object.ToFirstIdentifier());
                         break;
                     case OpenApiSpecVersion.OpenApi3_0 when isNullable && type.Value != JsonSchemaType.Null:
                         writer.WriteProperty(OpenApiConstants.Nullable, true);

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

@@ -159,7 +159,7 @@ internal static partial class OpenApiV3Deserializer
                 "type",
                 (o, n, _) => {
                     var type = n.GetScalarValue()?.ToJsonSchemaType();
-                    // so we don't loose the value from nullable
+                    // so we don't lose the value from nullable
                     if (o.Type.HasValue)
                         o.Type |= type;
                     else

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

@@ -286,7 +286,10 @@ internal static partial class OpenApiV31Deserializer
                         var nullable = bool.Parse(value);
                         if (nullable) // if nullable, convert type into an array of type(s) and null
                         {
-                            o.Type |= JsonSchemaType.Null;
+                            if (o.Type.HasValue)
+                                o.Type |= JsonSchemaType.Null;
+                            else
+                                o.Type = JsonSchemaType.Null;
                         }
                     }
                 }
@@ -392,8 +395,11 @@ public static IOpenApiSchema LoadSchema(ParseNode node, OpenApiDocument hostDocu
 
             if (schema.Extensions is not null && schema.Extensions.ContainsKey(OpenApiConstants.NullableExtension))
             {
-                var type = schema.Type;
-                schema.Type = type | JsonSchemaType.Null;
+                if (schema.Type.HasValue)
+                    schema.Type |= JsonSchemaType.Null;
+                else
+                    schema.Type = JsonSchemaType.Null;
+                
                 schema.Extensions.Remove(OpenApiConstants.NullableExtension);
             }
 

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiSchemaTests.cs

@@ -2,18 +2,19 @@
 // Licensed under the MIT license.
 
 using System.IO;
-using FluentAssertions;
-using Microsoft.OpenApi.Reader.V2;
-using Xunit;
-using Microsoft.OpenApi.Reader.ParseNodes;
-using Microsoft.OpenApi.Models;
-using Microsoft.OpenApi.Extensions;
 using System.Text.Json.Nodes;
-using System.Collections.Generic;
+using System.Threading.Tasks;
+using FluentAssertions;
 using FluentAssertions.Equivalency;
+using Microsoft.OpenApi.Extensions;
+using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Models.References;
+using Microsoft.OpenApi.Reader;
+using Microsoft.OpenApi.Reader.ParseNodes;
+using Microsoft.OpenApi.Reader.V2;
+using Microsoft.OpenApi.Tests;
 using Microsoft.OpenApi.Writers;
-using Microsoft.OpenApi.Models.Interfaces;
+using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests.V2Tests
 {
@@ -98,6 +99,7 @@ public void ParseSchemaWithEnumShouldSucceed()
                               .Excluding((IMemberInfo memberInfo) =>
                                     memberInfo.Path.EndsWith("Parent")));
         }
+
         [Fact]
         public void PropertiesReferenceShouldWork()
         {
@@ -152,5 +154,42 @@ public void PropertiesReferenceShouldWork()
             );
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(json), expected));
         }
+
+        [Fact]
+        public async Task SerializeSchemaWithNullableShouldSucceed()
+        {
+            // Arrange
+            var expected = @"type: string
+x-nullable: true";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithNullableExtension.yaml");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi2_0, new(), SettingsFixture.ReaderSettings);
+
+            var writer = new StringWriter();
+            schema.SerializeAsV2(new OpenApiYamlWriter(writer));
+            var schemaString = writer.ToString();
+
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
+
+        [Fact]
+        public async Task SerializeSchemaWithOnlyNullableShouldSucceed()
+        {
+            // Arrange
+            var expected = @"x-nullable: true";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithOnlyNullableExtension.yaml");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi2_0, new(), SettingsFixture.ReaderSettings);
+
+            var writer = new StringWriter();
+            schema.SerializeAsV2(new OpenApiYamlWriter(writer));
+            var schemaString = writer.ToString();
+
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V2Tests/Samples/OpenApiSchema/schemaWithNullableExtension.yaml

@@ -0,0 +1,2 @@
+type: string
+x-nullable: true

test/Microsoft.OpenApi.Readers.Tests/V2Tests/Samples/OpenApiSchema/schemaWithOnlyNullableExtension.yaml

@@ -0,0 +1 @@
+x-nullable: true

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

@@ -3,21 +3,22 @@
 
 using System.Collections.Generic;
 using System.IO;
+using System.Net.Http;
 using System.Text.Json.Nodes;
+using System.Threading.Tasks;
 using FluentAssertions;
-using Microsoft.OpenApi.Models;
+using FluentAssertions.Equivalency;
 using Microsoft.OpenApi.Extensions;
-using SharpYaml.Serialization;
-using Xunit;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Models.References;
 using Microsoft.OpenApi.Reader;
 using Microsoft.OpenApi.Reader.ParseNodes;
 using Microsoft.OpenApi.Reader.V3;
-using FluentAssertions.Equivalency;
-using Microsoft.OpenApi.Models.References;
-using System.Threading.Tasks;
-using System.Net.Http;
+using Microsoft.OpenApi.Tests;
+using Microsoft.OpenApi.Writers;
 using Microsoft.OpenApi.YamlReader;
-using Microsoft.OpenApi.Models.Interfaces;
+using SharpYaml.Serialization;
+using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests.V3Tests
 {
@@ -445,5 +446,42 @@ public async Task ParseExternalReferenceSchemaShouldSucceed()
 
             Assert.Equivalent(expectedComponents, components);
         }
+
+        [Fact]
+        public async Task SerializeSchemaWithNullableShouldSucceed()
+        {
+            // Arrange
+            var expected = @"type: string
+nullable: true";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithNullable.yaml");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi3_0, new(), SettingsFixture.ReaderSettings);
+
+            var writer = new StringWriter();
+            schema.SerializeAsV3(new OpenApiYamlWriter(writer));
+            var schemaString = writer.ToString();
+
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
+
+        [Fact]
+        public async Task SerializeSchemaWithOnlyNullableShouldSucceed()
+        {
+            // Arrange
+            var expected = @"nullable: true";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithOnlyNullable.yaml");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi3_0, new(), SettingsFixture.ReaderSettings);
+
+            var writer = new StringWriter();
+            schema.SerializeAsV3(new OpenApiYamlWriter(writer));
+            var schemaString = writer.ToString();
+
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/Samples/OpenApiSchema/schemaWithNullable.yaml

@@ -0,0 +1,2 @@
+type: string
+nullable: true

test/Microsoft.OpenApi.Readers.Tests/V3Tests/Samples/OpenApiSchema/schemaWithOnlyNullable.yaml

@@ -0,0 +1 @@
+nullable: true