NetCore 统一处理 webapi 返回null 转为“”

数据库中部分表字段允许空值，则代码中实体类对应的字段类型为可空类型nullable<>，如int?，datetime?，null值字段序列化返回的值都为null，前端对应字段赋值需要做null值判断，随想到全局把null替换为””

网上找的方案都是重写了defaultcontractresolver，但是该方法只能解决string类型null→””的问题，对其他可空类型无效。

研究了下git 上newtonsoft.json和.netcore mvc的源码

最终使用自定义jsonoutputformatter方式实现：

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 代码里面 修改

//services.addmvc().addjsonoptions(option => {
            //    //配置大小写问题，默认是首字母小写,该配置根据项目需要更改
            //    //option.serializersettings.contractresolver = new newtonsoft.json.serialization.defaultcontractresolver();
            //    //配置序列化时时间格式为时间戳
            //    //option.serializersettings.contractresolver = new nulltoemptystringresolver();
            //    option.serializersettings.dateformatstring = "yyyy-mm-dd hh:mm:ss";
            //});
            services.addmvc(config =>
            {
                var settings = new jsonserializersettings();
                //设置首字母小写，如果不需要 则改为 defaultcontractresolver
                settings.contractresolver = new camelcasepropertynamescontractresolver();
                //统一设置jsonresult中的日期格式    
                settings.dateformatstring = "yyyy-mm-dd hh:mm:ss";
                config.outputformatters.insert(0, new myjsonoutputformatter(settings));
            });