Address comments

Author: Youssef1313

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -547,14 +547,20 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalCollection(OpenApiConstants.AnyOf, effectiveAnyOf, callback);
 
             // oneOf
-            if (hasOneOfNullAndSingleEnumWith3_0 &&
-                effectiveOneOf![0] is OpenApiSchema { Enum.Count: > 0 } singleEffectiveOneOf)
+            if (hasOneOfNullAndSingleEnumWith3_0)
             {
-                writer.WriteRequiredCollection(OpenApiConstants.OneOf, effectiveOneOf, (writer, element) =>
-                {
-                    var clonedToMutateEnum = (OpenApiSchema)((OpenApiSchema)element).MemberwiseClone();
-                    clonedToMutateEnum.Enum = [.. clonedToMutateEnum.Enum!, null!];
-                    callback(writer, clonedToMutateEnum);
+                writer.WriteRequiredCollection(OpenApiConstants.OneOf, effectiveOneOf!, (writer, element) =>
+                {                    
+                    var clonedToMutateEnum = element.CreateShallowCopy();
+                    if (clonedToMutateEnum is OpenApiSchema { Enum: { } existingEnum } concreteCloned)
+                    {
+                        concreteCloned.Enum = [.. existingEnum, null!];
+                        callback(writer, clonedToMutateEnum);
+                    }
+                    else
+                    {
+                        callback(writer, element);
+                    }
                 });
             }
             else
@@ -1094,6 +1100,23 @@ private static (IList<IOpenApiSchema>? effective, JsonSchemaType? inferredType,
                     }
                     else if (schema.Enum is { Count: > 0 })
                     {
+                        foreach (var enumValue in schema.Enum)
+                        {
+                            if (enumValue is not null)
+                            {
+                                var currentType = enumValue.GetValueKind() switch
+                                {
+                                    JsonValueKind.Array => JsonSchemaType.Array,
+                                    JsonValueKind.String => JsonSchemaType.String,
+                                    JsonValueKind.Number => JsonSchemaType.Number,
+                                    JsonValueKind.True or JsonValueKind.False => JsonSchemaType.Boolean,
+                                    JsonValueKind.Null => (JsonSchemaType)0,
+                                    _ => JsonSchemaType.Object,
+                                };
+
+                                commonType |= currentType;
+                            }
+                        }
                         commonType |= JsonSchemaType.String;
                     }
                 }

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

@@ -1868,7 +1868,7 @@ public async Task SerializeNullableEnumWith3_0()
             // It's unclear if it's an issue of the validators or not, but it's safer to do it that way.
             var schema = CreateNullableEnumSchema();
             var result = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
-            Assert.Equal("""
+            var expected = """
                 {
                   "type": "string",
                   "oneOf": [
@@ -1882,7 +1882,9 @@ public async Task SerializeNullableEnumWith3_0()
                   ],
                   "nullable": true
                 }
-                """.ReplaceLineEndings(), result.ReplaceLineEndings());
+                """;
+
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(result)));
         }
 
         [Theory]
@@ -1892,7 +1894,7 @@ public async Task SerializeNullableEnumWith3_1_And_Later(OpenApiSpecVersion vers
         {
             var schema = CreateNullableEnumSchema();
             var result = await schema.SerializeAsJsonAsync(version);
-            Assert.Equal("""
+            var expected = """
                 {
                   "oneOf": [
                     {
@@ -1906,7 +1908,8 @@ public async Task SerializeNullableEnumWith3_1_And_Later(OpenApiSpecVersion vers
                     }
                   ]
                 }
-                """.ReplaceLineEndings(), result.ReplaceLineEndings());
+                """;
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(result)));
         }
 
         private OpenApiSchema CreateNullableEnumSchema()
@@ -1925,11 +1928,6 @@ private OpenApiSchema CreateNullableEnumSchema()
             return schema;
         }
 
-        private enum MyEnum
-        {
-            A, B
-        }
-
         internal class SchemaVisitor : OpenApiVisitorBase
         {
             public List<string> Titles = new();