fix/empty default

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -20,7 +20,7 @@
         /// <param name="writer">The writer.</param>
         /// <param name="name">The property name.</param>
         /// <param name="value">The property value.</param>
         public static void WriteProperty(this IOpenApiWriter writer, string name, string? value)
         {
             if (value == null)
             {
@@ -139,11 +139,11 @@
             T? value,
             Action<IOpenApiWriter, T> action)
         {
             if (value != null)
             {
-                if (value is IEnumerable values && !values.GetEnumerator().MoveNext())
+                if (value is IEnumerable values && value is not JsonArray && !values.GetEnumerator().MoveNext())
                 {
-                    return; // Don't render optional empty collections
+                    return; // Don't render optional empty collections except for the Default properties which are JsonArray
                 }
 
                 writer.WriteRequiredObject(name, value, action);
@@ -167,7 +167,7 @@
             Utils.CheckArgumentNull(action);
 
             writer.WritePropertyName(name);
             if (value != null)
             {
                 action(writer, value);
             }
@@ -262,7 +262,7 @@
         /// <param name="name">The property name.</param>
         /// <param name="elements">The map values.</param>
         /// <param name="action">The map element writer action.</param>
         public static void WriteRequiredMap(
             this IOpenApiWriter writer,
             string name,
             IDictionary<string, string>? elements,
@@ -421,7 +421,7 @@
             {
                 foreach (var item in elements)
                 {
                     if (item != null)
                     {
                         action(writer, item);
                     }
@@ -460,7 +460,7 @@
                 foreach (var item in elements)
                 {
                     writer.WritePropertyName(item.Key);
                     if (item.Value != null)
                     {
                         action(writer, item.Key, item.Value);
                     }

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiSchemaTests.cs

@@ -315,6 +315,40 @@ public void CloningSchemaWithExamplesAndEnumsShouldSucceed()
             Assert.Equivalent(6, clone.Default.GetValue<int>());
         }
 
+        [Fact]
+        public void DefaultEmptyCollectionShouldRoundTrip()
+        {
+            // Given
+            var serializedSchema =
+            """
+            {
+                "type": "array",
+                "items": {
+                    "type": "string",
+                    "default": []
+                }
+            }
+            """;
+            using var textWriter = new StringWriter();
+            var writer = new OpenApiJsonWriter(textWriter);
+
+            // When
+            var schema = OpenApiModelFactory.Parse<OpenApiSchema>(serializedSchema, OpenApiSpecVersion.OpenApi3_1, new(), out _, "json", SettingsFixture.ReaderSettings);
+
+            var deserializedArray = Assert.IsType<JsonArray>(schema.Items.Default);
+            Assert.Empty(deserializedArray);
+
+            schema.SerializeAsV31(writer);
+            var roundTrippedSchema = textWriter.ToString();
+
+            // Then
+            var parsedResult = JsonNode.Parse(roundTrippedSchema);
+            var parsedExpected = JsonNode.Parse(serializedSchema);
+            Assert.True(JsonNode.DeepEquals(parsedExpected, parsedResult));
+            var resultingArray = Assert.IsType<JsonArray>(parsedResult["items"]?["default"]);
+            Assert.Empty(resultingArray);
+        }
+
         [Fact]
         public async Task SerializeV31SchemaWithMultipleTypesAsV3Works()
         {