Kiota does not support array of errors

[0xced]

Let's say your OpenAPI operation error looks like that:

"404" : {
  "description" : "Status Code 404",
  "content" : {
    "application/json" : {
      "schema" : {
        "type" : "array",
        "items" : {
          "$ref" : "#/components/schemas/RestApiError"
        }
      }
    }
  }
}

And the RestApiError schema looks like that:

"RestApiError" : {
  "type" : "object",
  "properties" : {
    "errorCode" : {
      "type" : "string"
    },
    "message" : {
      "type" : "string"
    }
  }
}

The generated RestApiError class (which inherits from ApiException) can't be deserialized properly.

Here's a sample (actual) 404 error response:

[{"errorCode":"NOT_FOUND","message":"Some error message"}]

When thrown, the ErrorCode is null and the Message is Exception of type 'MySdk.Models.RestApiError' was thrown.

I'm not sure how Kiota should handle this situation where the error response is an array instead of an object and how the error class should be generated but currently the useful information (errorCode + message) is lost.

I was able to see the problem in the AssignFieldValues method of Microsoft.Kiota.Serialization.Json.JsonParseNode (which seems to be source generated)

private void AssignFieldValues<T>(T item) where T : IParsable
{
    if(_jsonNode.ValueKind != JsonValueKind.Object) return; // 👈 _jsonNode.ValueKind == JsonValueKind.Array ⇒ no deserialization occurs at all
    IDictionary<string, object>? itemAdditionalData = null;
    if(item is IAdditionalDataHolder holder)

[baywet]

Hi @0xced,
Thanks for using kiota and for reaching out.
This is a limitation of the current design.
See #2435 for additional details (although that issue is about enums, the limitations are the same)

[0xced]

It turns out that even though there could be several errors, the API only ever returns a single error in the array. So I was able to workaround this limitation by tweaking the CreateFromDiscriminatorValue method after the code is generated by kiota.

Generated code:

public static RestApiError CreateFromDiscriminatorValue(IParseNode parseNode) {
    _ = parseNode ?? throw new ArgumentNullException(nameof(parseNode));
    return new RestApiError();
}

Tweaked code:

public static RestApiError CreateFromDiscriminatorValue(IParseNode parseNode) {
    _ = parseNode ?? throw new ArgumentNullException(nameof(parseNode));
    var errors = parseNode.GetCollectionOfObjectValues<RestApiError>(_ => new RestApiError());
    return errors.FirstOrDefault() ?? new RestApiError();
}

I also added an override to the Message property in order to have a meaningful exception.

public override string Message => MessageEscaped ?? base.Message;

If the API ever returns multiple errors then at least I'll get the first error, which is already better than no error at all.