Fix example value of an empty array disappears after serialization

src/Microsoft.OpenApi/Models/OpenApiMediaType.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Writers;
@@ -76,7 +77,7 @@
             writer.WriteOptionalObject(OpenApiConstants.Example, Example, (w, e) => w.WriteAny(e));
 
             // examples
-            writer.WriteOptionalMap(OpenApiConstants.Examples, Examples, (w, e) => e.SerializeAsV3(w));
+            SerializeExamples(writer, Examples);
 
             // encoding
             writer.WriteOptionalMap(OpenApiConstants.Encoding, Encoding, (w, e) => e.SerializeAsV3(w));
@@ -94,5 +95,36 @@
         {
             // Media type does not exist in V2.
         }
+
+        private void SerializeExamples(IOpenApiWriter writer, IDictionary<string, OpenApiExample> examples)
+        {
+            /* Special case for writing out empty arrays as valid response examples
+            * Check if there is any example with an empty array as its value and set the flag `hasEmptyArray` to true
+            * */
+            var hasEmptyArray = examples.Values.Any(example =>
+                example.Value is OpenApiArray arr && arr.Count == 0
+            );
+
+            if (hasEmptyArray)
+            {
+                writer.WritePropertyName(OpenApiConstants.Examples);
+                writer.WriteStartObject();
+                foreach (var kvp in examples)
+                {
+                    if (kvp.Value.Value is OpenApiArray arr && arr.Count == 0)
+                    {
+                        writer.WritePropertyName(kvp.Key);
+                        writer.WriteStartObject();
+                        writer.WriteRequiredObject(OpenApiConstants.Value, kvp.Value.Value, (w, v) => w.WriteAny(v));
+                        writer.WriteEndObject();
+                    }
+                }
+                writer.WriteEndObject();
+            }
+            else
+            {
+                writer.WriteOptionalMap(OpenApiConstants.Examples, examples, (w, e) => e.SerializeAsV3(w));
+            }
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiMediaTypeTests.cs

@@ -4,9 +4,11 @@
 using System.IO;
 using FluentAssertions;
 using Microsoft.OpenApi.Any;
+using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Readers.ParseNodes;
 using Microsoft.OpenApi.Readers.V3;
+using Microsoft.OpenApi.Tests;
 using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests.V3Tests
@@ -77,5 +79,43 @@ public void ParseMediaTypeWithExamplesShouldSucceed()
                     }
                 });
         }
+
+        [Fact]
+        public void ParseMediaTypeWithEmptyArrayInExamplesWorks()
+        {
+            // Arrange
+            var expected = @"{
+  ""schema"": {
+    ""type"": ""array"",
+    ""items"": {
+      ""type"": ""object"",
+      ""properties"": {
+        ""id"": {
+          ""type"": ""string""
+        }
+      }
+    }
+  },
+  ""examples"": {
+    ""Success response - no results"": {
+      ""value"": [ ]
+    }
+  }
+}
+";
+            MapNode node;
+            using (var stream = Resources.GetStream(Path.Combine(SampleFolderPath, "examplesWithEmptyArray.json")))
+            {
+                node = TestHelper.CreateYamlMapNode(stream);
+            }
+
+            // Act
+            var mediaType = OpenApiV3Deserializer.LoadMediaType(node);
+            var serialized = mediaType.SerializeAsJson(OpenApiSpecVersion.OpenApi3_0);
+
+            // Assert
+            serialized.MakeLineBreaksEnvironmentNeutral()
+                .Should().BeEquivalentTo(expected.MakeLineBreaksEnvironmentNeutral());
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/Samples/OpenApiMediaType/examplesWithEmptyArray.json

@@ -0,0 +1,18 @@
+{
+  "schema": {
+    "type": "array",
+    "items": {
+      "type": "object",
+      "properties": {
+        "id": {
+          "type": "string"
+        }
+      }
+    }
+  },
+  "examples": {
+    "Success response - no results": {
+      "value": []
+    }
+  }
+}