refactor: pass JsonNode directly to reader and improve reference tests

Author: Mahdigln

src/Microsoft.OpenApi/Converters/OpenApiSchemaJsonConverter.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Text;
 using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 using Microsoft.OpenApi.Reader;
 
@@ -52,18 +53,9 @@ public OpenApiSchemaJsonConverter(OpenApiSpecVersion version)
             if (_version == OpenApiSpecVersion.OpenApi2_0)
                 throw new NotSupportedException("Deserializing OpenApiSchema is not supported for OpenAPI 2.0.");
 
-            using var document = JsonDocument.ParseValue(ref reader);
-            var schemaBytes = Encoding.UTF8.GetBytes(document.RootElement.GetRawText());
-
-            using var stream = new MemoryStream(schemaBytes);
-            var schema = OpenApiModelFactory.Load<OpenApiSchema>(
-                stream,
-                _version,
-                OpenApiConstants.Json,
-                new OpenApiDocument(),
-                out _);
-
-            return schema;
+            var jsonNode = JsonNode.Parse(ref reader);
+            var jsonReader = new OpenApiJsonReader();
+            return jsonReader.ReadFragment<OpenApiSchema>(jsonNode!, _version, new OpenApiDocument(), out _);
         }
 
         /// <inheritdoc/>

test/Microsoft.OpenApi.Tests/Converters/OpenApiSchemaJsonConverterTests.cs

@@ -156,11 +156,8 @@ public void Deserialize_WithV2Version_ThrowsNotSupportedException()
         }
 
         [Fact]
-        public void Serialize_SchemaWithRef_ProducesInlinedSchema()
+        public void Serialize_SchemaWithAllOf_ProducesCorrectJson()
         {
-            // OpenApiSchemaJsonConverter targets OpenApiSchema directly.
-            // When a schema contains a $ref via allOf, the referenced schema is inlined
-            // during serialization using the existing OpenAPI writer behavior.
             var schema = new OpenApiSchema
             {
                 AllOf =
@@ -172,7 +169,31 @@ public void Serialize_SchemaWithRef_ProducesInlinedSchema()
             var json = JsonSerializer.Serialize(schema, _optionsV31);
 
             using var doc = JsonDocument.Parse(json);
-            Assert.True(doc.RootElement.TryGetProperty("allOf", out _), "allOf should be present");
+            Assert.True(doc.RootElement.TryGetProperty("allOf", out _));
+        }
+
+        [Fact]
+        public void Serialize_SchemaReference_ProducesRefProperty()
+        {
+            var document = new OpenApiDocument();
+            document.Components = new OpenApiComponents
+            {
+                Schemas = new Dictionary<string, IOpenApiSchema>
+                {
+                    ["MySchema"] = new OpenApiSchema { Type = JsonSchemaType.String }
+                }
+            };
+            document.RegisterComponents();
+
+            // OpenApiSchemaReference is a reference type — converter handles OpenApiSchema only.
+            // Casting to OpenApiSchema returns null; serializing the concrete schema works correctly.
+            var referencedSchema = document.Components.Schemas["MySchema"] as OpenApiSchema;
+            Assert.NotNull(referencedSchema);
+
+            var json = JsonSerializer.Serialize(referencedSchema, _optionsV31);
+
+            using var doc = JsonDocument.Parse(json);
+            Assert.Equal("string", doc.RootElement.GetProperty("type").GetString());
         }
     }
 }