NetCore 統一處理 webapi 返回null 轉為“”

資料庫中部分表欄位允許空值，則代碼中實體類對應的欄位類型為可空類型Nullable<>，如int?，DateTime?，null值欄位序列化返回的值都為null，前端對應欄位賦值需要做null值判斷，隨想到全局把null替換為""

網上找的方案都是重寫了DefaultContractResolver，但是該方法只能解決string類型null→""的問題，對其他可空類型無效。

研究了下git 上Newtonsoft.Json和.netcore MVC的源碼

最終使用自定義JsonOutputFormatter方式實現：

public class MyJsonOutputFormatter : JsonOutputFormatter
        {
            public MyJsonOutputFormatter(JsonSerializerSettings serializerSettings) : base(serializerSettings, ArrayPool<char>.Shared)
            {
            }
            public new JsonSerializerSettings SerializerSettings => base.SerializerSettings;
            protected override JsonWriter CreateJsonWriter(TextWriter writer)
            {
                if (writer == null)
                {
                    throw new ArgumentNullException(nameof(writer));
                }
                var jsonWriter = new NullJsonWriter(writer)
                {
                    ArrayPool = new JsonArrayPool<char>(ArrayPool<char>.Shared),
                    CloseOutput = false,
                    AutoCompleteOnClose = false
                };
                return jsonWriter;
            }
        }
        public class NullJsonWriter : JsonTextWriter
        {
            public NullJsonWriter(TextWriter textWriter):base(textWriter)
            {
            }
            public override void WriteNull()
            {
                this.WriteValue(String.Empty);
            }
        }

在 Startup 代碼裡面 修改

//services.AddMvc().AddJsonOptions(option => {
            //    //配置大小寫問題，預設是首字母小寫,該配置根據項目需要更改
            //    //option.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.DefaultContractResolver();
            //    //配置序列化時時間格式為時間戳
            //    //option.SerializerSettings.ContractResolver = new NullToEmptyStringResolver();
            //    option.SerializerSettings.DateFormatString = "yyyy-MM-dd HH:mm:ss";
            //});
            services.AddMvc(config =>
            {
                var settings = new JsonSerializerSettings();
                //設置首字母小寫，如果不需要 則改為 DefaultContractResolver
                settings.ContractResolver = new CamelCasePropertyNamesContractResolver();
                //統一設置JsonResult中的日期格式    
                settings.DateFormatString = "yyyy-MM-dd HH:mm:ss";
                config.OutputFormatters.Insert(0, new MyJsonOutputFormatter(settings));
            });