Feat: Serialize const prop from 3.1 schemas as single enum

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -264,7 +264,7 @@
         /// Initializes a copy of <see cref="IOpenApiSchema"/> object
         /// </summary>
         /// <param name="schema">The schema object to copy from.</param>
         internal OpenApiSchema(IOpenApiSchema schema)
         {
             Utils.CheckArgumentNull(schema);
             Title = schema.Title ?? Title;
@@ -336,23 +336,23 @@
             SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer));
         }
 
         private static void SerializeBounds(IOpenApiWriter writer, OpenApiSpecVersion version, string propertyName, string exclusivePropertyName, string isExclusivePropertyName, string? value, string? exclusiveValue, bool? isExclusiveValue)
         {
             if (version >= OpenApiSpecVersion.OpenApi3_1)
             {
                 if (!string.IsNullOrEmpty(exclusiveValue) && exclusiveValue is not null)
                 {
                     // was explicitly set in the document or object model
                     writer.WritePropertyName(exclusivePropertyName);
                     writer.WriteRaw(exclusiveValue);
                 }
                 else if (isExclusiveValue == true && !string.IsNullOrEmpty(value) && value is not null)
                 {
                     // came from parsing an old document
                     writer.WritePropertyName(exclusivePropertyName);
                     writer.WriteRaw(value);
                 }
                 else if (!string.IsNullOrEmpty(value) && value is not null)
                 {
                     // was explicitly set in the document or object model
                     writer.WritePropertyName(propertyName);
@@ -361,14 +361,14 @@
             }
             else
             {
                 if (!string.IsNullOrEmpty(exclusiveValue) && exclusiveValue is not null)
                 {
                     // was explicitly set in a new document being downcast or object model
                     writer.WritePropertyName(propertyName);
                     writer.WriteRaw(exclusiveValue);
                     writer.WriteProperty(isExclusivePropertyName, true);
                 }
                 else if (!string.IsNullOrEmpty(value) && value is not null)
                 {
                     // came from parsing an old document, we're just mirroring the information
                     writer.WritePropertyName(propertyName);
@@ -430,14 +430,19 @@
             // required
             writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) =>
             {
                 if (!string.IsNullOrEmpty(s) && s is not null)
                 {
                     w.WriteValue(s);
                 }
             });
 
             // enum
-            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (nodeWriter, s) => nodeWriter.WriteAny(s));
+            var enumValue = (Enum == null || Enum.Count == 0)
+                && !string.IsNullOrEmpty(Const) 
+                && version < OpenApiSpecVersion.OpenApi3_1
+                ? new List<JsonNode> { JsonValue.Create(Const)! }
+                : Enum;
+            writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // type
             SerializeTypeProperty(writer, version);
@@ -618,7 +623,7 @@
         /// <param name="writer">The open api writer.</param>
         /// <param name="parentRequiredProperties">The list of required properties in parent schema.</param>
         /// <param name="propertyName">The property name that will be serialized.</param>
         private void SerializeAsV2(
             IOpenApiWriter writer,
             ISet<string>? parentRequiredProperties,
             string? propertyName)
@@ -687,7 +692,10 @@
             });
 
             // enum
-            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (w, s) => w.WriteAny(s));
+            var enumValue = (Enum == null || Enum.Count == 0) && !string.IsNullOrEmpty(Const)
+                ? new List<JsonNode> { JsonValue.Create(Const)! }
+                : Enum;
+            writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // items
             writer.WriteOptionalObject(OpenApiConstants.Items, Items, (w, s) => s.SerializeAsV2(w));

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

@@ -643,6 +643,46 @@ public async Task WriteAsItemsPropertiesDoesNotWriteNull()
             """;
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
         }
+        [Fact]
+        public async Task SerializeConstAsEnumV30()
+        {
+            // Arrange
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.String,
+                Const = "foo"
+            };
+
+
+            // Act
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+
+            // Assert
+            var v3Node = JsonNode.Parse(actual);
+            Assert.NotNull(v3Node);
+            Assert.True(v3Node["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.False(v3Node.AsObject().ContainsKey("const"));
+        }
+
+        [Fact]
+        public async Task SerializeConstAsEnumV20()
+        {
+            // Arrange
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.String,
+                Const = "foo"
+            };
+
+            // Act
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Assert
+            var v2Node = JsonNode.Parse(actual);
+            Assert.NotNull(v2Node);
+            Assert.True(v2Node["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.False(v2Node.AsObject().ContainsKey("const"));
+        }
 
 
         internal class SchemaVisitor : OpenApiVisitorBase