Fix ExtensionData merge behavior and add enum support for JsonSchemaExtensionDataAttribute

src/NJsonSchema.Annotations/JsonSchemaExtensionDataAttribute.cs

@@ -11,7 +11,7 @@ namespace NJsonSchema.Annotations;
 /// <summary>Adds an extension data property to a class or property.</summary>
 /// <seealso cref="System.Attribute" />
 [AttributeUsage(
-    AttributeTargets.Class | AttributeTargets.Property | AttributeTargets.Parameter | AttributeTargets.ReturnValue,
+    AttributeTargets.Class | AttributeTargets.Enum | AttributeTargets.Property | AttributeTargets.Parameter | AttributeTargets.ReturnValue,
     AllowMultiple = true)]
 public class JsonSchemaExtensionDataAttribute : Attribute, IJsonSchemaExtensionDataAttribute
 {

src/NJsonSchema.Tests/Serialization/ExtensionDataTests.cs

@@ -95,6 +95,50 @@ public async Task When_extension_data_attribute_is_used_on_class_then_extension_
             Assert.Equal(123, schema.ExtensionData["MyClass"]);
         }
 
+        [JsonSchemaExtensionData("x-tag", "my-enum")]
+        public enum MyTaggedEnum
+        {
+            Value1,
+            Value2
+        }
+
+        [Fact]
+        public async Task When_extension_data_attribute_is_used_on_enum_then_extension_data_property_is_set()
+        {
+            // Act
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<MyTaggedEnum>();
+
+            // Assert
+            Assert.Equal("my-enum", schema.ExtensionData["x-tag"]);
+        }
+
+        [JsonSchemaExtensionData("x-attribute", "from-attribute")]
+        public class MyMergeTest { }
+
+        private class AddExtensionDataProcessor : NJsonSchema.Generation.ISchemaProcessor
+        {
+            public void Process(NJsonSchema.Generation.SchemaProcessorContext context)
+            {
+                context.Schema.ExtensionData ??= new Dictionary<string, object>();
+                context.Schema.ExtensionData["x-processor"] = "from-processor";
+            }
+        }
+
+        [Fact]
+        public async Task When_schema_processor_and_extension_data_attribute_are_used_together_then_both_are_preserved()
+        {
+            // Arrange
+            var settings = new NJsonSchema.NewtonsoftJson.Generation.NewtonsoftJsonSchemaGeneratorSettings();
+            settings.SchemaProcessors.Add(new AddExtensionDataProcessor());
+
+            // Act
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<MyMergeTest>(settings);
+
+            // Assert
+            Assert.Equal("from-attribute", schema.ExtensionData["x-attribute"]);
+            Assert.Equal("from-processor", schema.ExtensionData["x-processor"]);
+        }
+
         [Fact]
         public async Task When_extension_data_attribute_is_used_on_property_then_extension_data_property_is_set()
         {

src/NJsonSchema/Generation/JsonSchemaGenerator.cs

@@ -1382,14 +1382,12 @@ private static void ApplyTypeExtensionDataAttributes(JsonSchema schema, IJsonSch
         {
             if (extensionAttributes.Length > 0)
             {
-                var extensionData = new Dictionary<string, object?>();
+                schema.ExtensionData ??= new Dictionary<string, object?>();
                 foreach (var kvp in extensionAttributes
                     .SelectMany(attribute => attribute.ExtensionData))
                 {
-                    extensionData[kvp.Key] = kvp.Value;
+                    schema.ExtensionData[kvp.Key] = kvp.Value;
                 }
-
-                schema.ExtensionData = extensionData;
             }
         }
     }