OpenAPI: Fix Circular reference in specific order gives empty schema

src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs

@@ -337,7 +337,7 @@ internal static IOpenApiSchema ResolveReferenceForSchema(OpenApiDocument documen
             if (schema.Metadata.TryGetValue(OpenApiConstants.SchemaId, out var schemaId) &&
                 schemaId is string schemaIdString)
             {
-                return document.AddOpenApiSchemaByReference(schemaIdString, schema);
+                return new OpenApiSchemaReference(schemaIdString, document);
             }
             var relativeSchemaId = $"#/components/schemas/{rootSchemaId}{refIdString.Replace("#", string.Empty)}";
             return new OpenApiSchemaReference(relativeSchemaId, document);

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.SchemaReferences.cs

@@ -0,0 +1,138 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Routing;
+
+public partial class OpenApiSchemaServiceTests : OpenApiDocumentServiceTestBase
+{
+    [Fact]
+    public async Task SchemaReferences_HandlesCircularReferencesRegardlessOfPropertyOrder_SelfFirst()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelSelfFirst dto) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var schema = document.Components.Schemas["DirectCircularModelSelfFirst"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("self", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelSelfFirst", reference.Reference.ReferenceV3);
+                },
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
+    [Fact]
+    public async Task SchemaReferences_HandlesCircularReferencesRegardlessOfPropertyOrder_SelfLast()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelSelfLast dto) => { });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var schema = document.Components.Schemas["DirectCircularModelSelfLast"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                },
+                property =>
+                {
+                    Assert.Equal("self", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelSelfLast", reference.Reference.ReferenceV3);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
+    [Fact]
+    public async Task SchemaReferences_HandlesCircularReferencesRegardlessOfPropertyOrder_MultipleSelf()
+    {
+        var builder = CreateBuilder();
+        builder.MapPost("/", (DirectCircularModelMultiple dto) => { });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var schema = document.Components.Schemas["DirectCircularModelMultiple"];
+            Assert.Equal(JsonSchemaType.Object, schema.Type);
+            Assert.Collection(schema.Properties,
+                property =>
+                {
+                    Assert.Equal("selfFirst", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelMultiple", reference.Reference.ReferenceV3);
+                },
+                property =>
+                {
+                    Assert.Equal("referenced", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                },
+                property =>
+                {
+                    Assert.Equal("selfLast", property.Key);
+                    var reference = Assert.IsType<OpenApiSchemaReference>(property.Value);
+                    Assert.Equal("#/components/schemas/DirectCircularModelMultiple", reference.Reference.ReferenceV3);
+                });
+
+            // Verify that it does not result in an empty schema for a referenced schema
+            var referencedSchema = document.Components.Schemas["ReferencedModel"];
+            Assert.NotEmpty(referencedSchema.Properties);
+            var idProperty = Assert.Single(referencedSchema.Properties);
+            Assert.Equal("id", idProperty.Key);
+            var idPropertySchema = Assert.IsType<OpenApiSchema>(idProperty.Value);
+            Assert.Equal(JsonSchemaType.Integer, idPropertySchema.Type);
+        });
+    }
+
+    private class DirectCircularModelSelfFirst
+    {
+        public DirectCircularModelSelfFirst Self { get; set; } = null!;
+        public ReferencedModel Referenced { get; set; } = null!;
+    }
+
+    private class DirectCircularModelSelfLast
+    {
+        public ReferencedModel Referenced { get; set; } = null!;
+        public DirectCircularModelSelfLast Self { get; set; } = null!;
+    }
+
+    private class DirectCircularModelMultiple
+    {
+        public DirectCircularModelMultiple SelfFirst { get; set; } = null!;
+        public ReferencedModel Referenced { get; set; } = null!;
+        public DirectCircularModelMultiple SelfLast { get; set; } = null!;
+    }
+
+    private class ReferencedModel
+    {
+        public int Id { get; set; }
+    }
+}