resolve PR comments

Author: Michael-Wamae

docs/upgrade-guide-2.md

@@ -487,10 +487,24 @@ The `SerializeAs()` method simplifies serialization scenarios, making it easier
 
 ```csharp
 OpenApiDocument document = new OpenApiDocument();
-string json = document.SerializeAs(OpenApiSpecVersion.OpenApi3_0, OpenApiFormat.Json);
+string json = document.SerializeAs(OpenApiSpecVersion.OpenApi3_0, OpenApiConstants.Json);
 
 ```
 
+### Use OpenApiConstants string Instead of OpenApiFormat Enum
+
+OpenApiConstants are now used instead of OpenApiFormat enums.
+
+**Example:**
+
+```csharp
+// Before (1.6)
+var outputString = openApiDocument.Serialize(OpenApiSpecVersion.OpenApi2_0, OpenApiFormat.Json); 
+
+// After (2.0)
+var outputString = openApiDocument.Serialize(OpenApiSpecVersion.OpenApi2_0, OpenApiConstants.Json);
+```
+
 ### Bug Fixes
 
 ## Serialization of References

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -202,8 +202,9 @@ private static async Task WriteOpenApiAsync(HidiOptions options, string openApiF
                     InlineLocalReferences = options.InlineLocal,
                     InlineExternalReferences = options.InlineExternal
                 };
-
-                IOpenApiWriter writer = openApiFormat switch
+#pragma warning disable CA1308
+                IOpenApiWriter writer = openApiFormat.ToLowerInvariant() switch
+#pragma warning restore CA1308
                 {
                     OpenApiConstants.Json => options.TerseOutput ? new(textWriter, settings, options.TerseOutput) : new OpenApiJsonWriter(textWriter, settings, false),
                     OpenApiConstants.Yaml => new OpenApiYamlWriter(textWriter, settings),

src/Microsoft.OpenApi/Extensions/OpenApiSerializableExtensions.cs

@@ -91,7 +91,7 @@ public static Task SerializeAsync<T>(
 
             var streamWriter = new FormattingStreamWriter(stream, CultureInfo.InvariantCulture);
 
-            IOpenApiWriter writer = format switch
+            IOpenApiWriter writer = format.ToLowerInvariant() switch
             {
                 OpenApiConstants.Json => new OpenApiJsonWriter(streamWriter, settings, false),
                 OpenApiConstants.Yaml => new OpenApiYamlWriter(streamWriter, settings),