Asp.Net Core Mvc上Json序列化首字母大小写的问题

在mvc中返回Json格式数据，用Return Json()时，序列化后的Json串首字母全变成了小写。

第一种解决方案

用 return Json(new { code = 0, msg = "", count = datas.Count, data = datas });

        public IServiceProvider ConfigureServices(IServiceCollection services)
        {
            services.AddMvc().AddJsonOptions(options => { options.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.DefaultContractResolver(); });
            //services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
            //解决Return Json序列化后首字母自动变成小写的问题
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1).AddJsonOptions(options => { options.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.DefaultContractResolver(); });
            AutoMapperConfig.Register();
            return services.AddDependenciesInjection();
        }

第二种方案  Return Content()

先将要返回的数据序列化成Json串

var toJson = new { code = 0, msg = "", count = datas.Count, data = datas };

string strJson = Newtonsoft.Json.JsonConvert.SerializeObject(toJson);

Return Content(strJson)