fix(schema): serialize compatibility examples from examples list

Author: Copilot

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -615,7 +615,10 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalObject(OpenApiConstants.ExternalDocs, ExternalDocs, callback);
 
             // example
-            writer.WriteOptionalObject(OpenApiConstants.Example, Example, (w, e) => w.WriteAny(e));
+            writer.WriteOptionalObject(
+                OpenApiConstants.Example,
+                version < OpenApiSpecVersion.OpenApi3_1 ? GetCompatibilityExample() : Example,
+                (w, e) => w.WriteAny(e));
 
             // deprecated
             writer.WriteProperty(OpenApiConstants.Deprecated, Deprecated, false);
@@ -741,7 +744,7 @@ private void WriteV3CompatibilityKeywords(IOpenApiWriter writer, Action<IOpenApi
             writer.WriteOptionalObject(OpenApiConstants.IfExtension, If, callback);
             writer.WriteOptionalObject(OpenApiConstants.ThenExtension, Then, callback);
             writer.WriteOptionalObject(OpenApiConstants.ElseExtension, Else, callback);
-            writer.WriteOptionalCollection(OpenApiConstants.JsonSchemaExamplesExtension, Examples, (nodeWriter, s) => nodeWriter.WriteAny(s));
+            writer.WriteOptionalCollection(OpenApiConstants.JsonSchemaExamplesExtension, GetCompatibilityExamplesExtension(), (nodeWriter, s) => nodeWriter.WriteAny(s));
         }
 
         internal void WriteAsItemsProperties(IOpenApiWriter writer)
@@ -955,7 +958,7 @@ private void SerializeAsV2(
             writer.WriteOptionalObject(OpenApiConstants.ExternalDocs, ExternalDocs, (w, s) => s.SerializeAsV2(w));
 
             // example
-            writer.WriteOptionalObject(OpenApiConstants.Example, Example, (w, e) => w.WriteAny(e));
+            writer.WriteOptionalObject(OpenApiConstants.Example, GetCompatibilityExample(), (w, e) => w.WriteAny(e));
 
             // x-nullable extension
             SerializeNullable(writer, OpenApiSpecVersion.OpenApi2_0);
@@ -986,7 +989,7 @@ private void SerializeAsV2(
                 writer.WriteOptionalMap(OpenApiConstants.PatternPropertiesExtension, PatternProperties, (w, s) => s.SerializeAsV2(w));
             }
 
-            writer.WriteOptionalCollection(OpenApiConstants.JsonSchemaExamplesExtension, Examples, (nodeWriter, s) => nodeWriter.WriteAny(s));
+            writer.WriteOptionalCollection(OpenApiConstants.JsonSchemaExamplesExtension, GetCompatibilityExamplesExtension(), (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // extensions
             writer.WriteExtensions(Extensions, OpenApiSpecVersion.OpenApi2_0);
@@ -1022,6 +1025,26 @@ private bool TrySerializeTypeProperty(IOpenApiWriter writer, OpenApiSpecVersion
             return false;
         }
 
+        private JsonNode? GetCompatibilityExample()
+        {
+            return Example ?? Examples?.FirstOrDefault();
+        }
+
+        private IEnumerable<JsonNode>? GetCompatibilityExamplesExtension()
+        {
+            if (Examples is null || Examples.Count == 0)
+            {
+                return null;
+            }
+
+            if (Example is not null)
+            {
+                return Examples;
+            }
+
+            return Examples.Count > 1 ? Examples.Skip(1) : null;
+        }
+
         private static bool IsPowerOfTwo(int x)
         {
             return x != 0 && (x & (x - 1)) == 0;

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

@@ -1749,12 +1749,35 @@ public async Task SerializePatternPropertiesAsExtensionInEarlierVersions(OpenApi
         [Theory]
         [InlineData(OpenApiSpecVersion.OpenApi2_0)]
         [InlineData(OpenApiSpecVersion.OpenApi3_0)]
-        public async Task SerializeExamplesAsExtensionInEarlierVersions(OpenApiSpecVersion version)
+        public async Task SerializeSingleExampleAsExamplePropertyInEarlierVersionsWhenExampleIsUnset(OpenApiSpecVersion version)
         {
             var expected = """
                 {
+                  "example": "example value"
+                }
+                """;
+            var schema = new OpenApiSchema
+            {
+                Examples =
+                [
+                    JsonValue.Create("example value")!
+                ]
+            };
+
+            var actual = await schema.SerializeAsJsonAsync(version);
+
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi2_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        public async Task SerializeMultipleExamplesAsExampleAndExtensionInEarlierVersionsWhenExampleIsUnset(OpenApiSpecVersion version)
+        {
+            var expected = """
+                {
+                  "example": "example value",
                   "x-jsonschema-examples": [
-                    "example value",
                     42
                   ]
                 }