Do not mark require property as read only

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -374,6 +374,32 @@ public void SerializeAsV3WithoutReference(IOpenApiWriter writer)
         /// Serialize <see cref="OpenApiSchema"/> to Open Api v2.0
         /// </summary>
         public void SerializeAsV2(IOpenApiWriter writer)
+        {
+            SerializeAsV2(writer: writer, parentRequiredProperties: new List<string>(), propertyName: null);
+        }
+
+        /// <summary>
+        /// Serialize to OpenAPI V2 document without using reference.
+        /// </summary>
+        public void SerializeAsV2WithoutReference(IOpenApiWriter writer)
+        {
+            SerializeAsV2WithoutReference(
+                writer: writer,
+                parentRequiredProperties: new List<string>(),
+                propertyName: null);
+        }
+
+        /// <summary>
+        /// Serialize <see cref="OpenApiSchema"/> to Open Api v2.0 and handles not marking the provided property 
+        /// as readonly if its included in the provided list of required properties of parent schema.
+        /// </summary>
+        /// <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>
+        internal void SerializeAsV2(
+            IOpenApiWriter writer,
+            IList<string> parentRequiredProperties,
+            string propertyName)
         {
             if (writer == null)
             {
@@ -386,16 +412,28 @@ public void SerializeAsV2(IOpenApiWriter writer)
                 return;
             }
 
-            SerializeAsV2WithoutReference(writer);
+            if (parentRequiredProperties == null)
+            {
+                parentRequiredProperties = new List<string>();
+            }
+
+            SerializeAsV2WithoutReference(writer, parentRequiredProperties, propertyName);
         }
 
         /// <summary>
-        /// Serialize to OpenAPI V2 document without using reference.
+        /// Serialize to OpenAPI V2 document without using reference and handles not marking the provided property 
+        /// as readonly if its included in the provided list of required properties of parent schema.
         /// </summary>
-        public void SerializeAsV2WithoutReference(IOpenApiWriter writer)
+        /// <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>
+        internal void SerializeAsV2WithoutReference(
+            IOpenApiWriter writer,
+            IList<string> parentRequiredProperties,
+            string propertyName)
         {
             writer.WriteStartObject();
-            WriteAsSchemaProperties(writer);
+            WriteAsSchemaProperties(writer, parentRequiredProperties, propertyName);
             writer.WriteEndObject();
         }
 
@@ -463,7 +501,10 @@ internal void WriteAsItemsProperties(IOpenApiWriter writer)
             writer.WriteExtensions(Extensions);
         }
 
-        internal void WriteAsSchemaProperties(IOpenApiWriter writer)
+        internal void WriteAsSchemaProperties(
+            IOpenApiWriter writer,
+            IList<string> parentRequiredProperties,
+            string propertyName)
         {
             if (writer == null)
             {
@@ -537,7 +578,8 @@ internal void WriteAsSchemaProperties(IOpenApiWriter writer)
             writer.WriteOptionalCollection(OpenApiConstants.AllOf, AllOf, (w, s) => s.SerializeAsV2(w));
 
             // properties
-            writer.WriteOptionalMap(OpenApiConstants.Properties, Properties, (w, s) => s.SerializeAsV2(w));
+            writer.WriteOptionalMap(OpenApiConstants.Properties, Properties, (w, key, s) =>
+                s.SerializeAsV2(w, Required, key));
 
             // additionalProperties
             writer.WriteOptionalObject(
@@ -549,7 +591,12 @@ internal void WriteAsSchemaProperties(IOpenApiWriter writer)
             writer.WriteProperty(OpenApiConstants.Discriminator, Discriminator?.PropertyName);
 
             // readOnly
-            writer.WriteProperty(OpenApiConstants.ReadOnly, ReadOnly, false);
+            // In V2 schema if a property is part of required properties of parent schema,
+            // it cannot be marked as readonly.
+            if (!parentRequiredProperties.Contains(propertyName))
+            {
+                writer.WriteProperty(name: OpenApiConstants.ReadOnly, value: ReadOnly, defaultValue: false);
+            }
 
             // xml
             writer.WriteOptionalObject(OpenApiConstants.Xml, Xml, (w, s) => s.SerializeAsV2(w));

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

@@ -160,6 +160,60 @@ public class OpenApiSchemaTests
             }
         };
 
+        public static OpenApiSchema AdvancedSchemaWithRequiredPropertiesObject = new OpenApiSchema
+        {
+            Title = "title1",
+            Required = new List<string>(){ "property1" },
+            Properties = new Dictionary<string, OpenApiSchema>
+            {
+                ["property1"] = new OpenApiSchema
+                {
+                    Required = new List<string>() { "property3" },
+                    Properties = new Dictionary<string, OpenApiSchema>
+                    {
+                        ["property2"] = new OpenApiSchema
+                        {
+                            Type = "integer"
+                        },
+                        ["property3"] = new OpenApiSchema
+                        {
+                            Type = "string",
+                            MaxLength = 15,
+                            ReadOnly = true
+                        }
+                    },
+                    ReadOnly = true,
+                },
+                ["property4"] = new OpenApiSchema
+                {
+                    Properties = new Dictionary<string, OpenApiSchema>
+                    {
+                        ["property5"] = new OpenApiSchema
+                        {
+                            Properties = new Dictionary<string, OpenApiSchema>
+                            {
+                                ["property6"] = new OpenApiSchema
+                                {
+                                    Type = "boolean"
+                                }
+                            }
+                        },
+                        ["property7"] = new OpenApiSchema
+                        {
+                            Type = "string",
+                            MinLength = 2
+                        }
+                    },
+                    ReadOnly = true,
+                },
+            },
+            Nullable = true,
+            ExternalDocs = new OpenApiExternalDocs
+            {
+                Url = new Uri("http://example.com/externalDocs")
+            }
+        };
+
         private readonly ITestOutputHelper _output;
 
         public OpenApiSchemaTests(ITestOutputHelper output)
@@ -363,5 +417,64 @@ public void SerializeReferencedSchemaAsV3JsonWorks()
             expected = expected.MakeLineBreaksEnvironmentNeutral();
             actual.Should().Be(expected);
         }
+
+        [Fact]
+        public void SerializeSchemaWRequiredPropertiesAsV2JsonWorks()
+        {
+            // Arrange
+            var outputStringWriter = new StringWriter();
+            var writer = new OpenApiJsonWriter(outputStringWriter);
+            var expected = @"{
+  ""title"": ""title1"",
+  ""required"": [
+    ""property1""
+  ],
+  ""properties"": {
+    ""property1"": {
+      ""required"": [
+        ""property3""
+      ],
+      ""properties"": {
+        ""property2"": {
+          ""type"": ""integer""
+        },
+        ""property3"": {
+          ""maxLength"": 15,
+          ""type"": ""string""
+        }
+      }
+    },
+    ""property4"": {
+      ""properties"": {
+        ""property5"": {
+          ""properties"": {
+            ""property6"": {
+              ""type"": ""boolean""
+            }
+          }
+        },
+        ""property7"": {
+          ""minLength"": 2,
+          ""type"": ""string""
+        }
+      },
+      ""readOnly"": true
+    }
+  },
+  ""externalDocs"": {
+    ""url"": ""http://example.com/externalDocs""
+  }
+}";
+
+            // Act
+            AdvancedSchemaWithRequiredPropertiesObject.SerializeAsV2(writer);
+            writer.Flush();
+            var actual = outputStringWriter.GetStringBuilder().ToString();
+
+            // Assert
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
+        }
     }
 }