Retrieve format property during V2 serialization

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
 using System.Collections.Generic;
@@ -630,6 +630,10 @@ internal void WriteAsSchemaProperties(
             }
 
             // format
+            Format ??= AllOf?.FirstOrDefault(static x => x.Format != null)?.Format ??
+                    AnyOf?.FirstOrDefault(static x => x.Format != null)?.Format ??
+                    OneOf?.FirstOrDefault(static x => x.Format != null)?.Format;
+
             writer.WriteProperty(OpenApiConstants.Format, Format);
 
             // title
@@ -695,7 +699,7 @@ internal void WriteAsSchemaProperties(
             // allOf
             writer.WriteOptionalCollection(OpenApiConstants.AllOf, AllOf, (w, s) => s.SerializeAsV2(w));
 
-            // If there isn't already an AllOf, and the schema contains a oneOf or anyOf write an allOf with the first
+            // If there isn't already an allOf, and the schema contains a oneOf or anyOf write an allOf with the first
             // schema in the list as an attempt to guess at a graceful downgrade situation.
             if (AllOf == null || AllOf.Count == 0)
             {

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

@@ -422,5 +422,46 @@ public async Task SerializeSchemaWRequiredPropertiesAsV2JsonWorksAsync(bool prod
             // Assert
             await Verifier.Verify(actual).UseParameters(produceTerseOutput);
         }
+
+        [Fact]
+        public void SerializeAsV2ShouldSetFormatPropertyInParentSchemaIfPresentInChildrenSchema()
+        {
+            // Arrange
+            var schema = new OpenApiSchema()
+            {
+                OneOf = new List<OpenApiSchema>
+                {
+                    new OpenApiSchema
+                    { 
+                        Type = "number",
+                        Format = "decimal"
+                    },
+                    new OpenApiSchema { Type = "string" },
+                }
+            };
+
+            var outputStringWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var openApiJsonWriter = new OpenApiJsonWriter(outputStringWriter, new OpenApiJsonWriterSettings { Terse = false });
+
+            // Act
+            // Serialize as V2
+            schema.SerializeAsV2(openApiJsonWriter);
+            openApiJsonWriter.Flush();
+
+            var v2Schema = outputStringWriter.GetStringBuilder().ToString().MakeLineBreaksEnvironmentNeutral();
+
+            var expectedV2Schema = @"{
+  ""format"": ""decimal"",
+  ""allOf"": [
+    {
+      ""format"": ""decimal"",
+      ""type"": ""number""
+    }
+  ]
+}".MakeLineBreaksEnvironmentNeutral();
+
+            // Assert
+            Assert.Equal(expectedV2Schema, v2Schema);
+        }
     }
 }