ABP A possible object cycle was detected.

問題

在 Abp AppService 中回傳的物件如果有 recursion 的狀況，像是取回 PermissionGroupDefinition ，會包含 List<PermissionDefinition> ，而 PermissionDefinition 又會包含 List<PermissionDefinition>，所以就會出現以下的錯誤，

[ERR] A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles. Path: $.Permissions.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.
System.Text.Json.JsonException: A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles. Path: $.Permissions.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.Parent.Children.
   at System.Text.Json.ThrowHelper.ThrowJsonException_SerializerCycleDetected(Int32 maxDepth)
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)

解法

參考Adding configuration for JSON serializer providers doesn’t work，在 Web 專案的 Module 檔案中，加入以下的設定就可以正常運作。

Configure<Microsoft.AspNetCore.Mvc.JsonOptions>(options =>
{
    options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles;
});

參考資源

Adding configuration for JSON serializer providers doesn’t work