Step 1: install microsoft.NewtonsoftJson

Go to tools -> NutGet Package Manager -> Manage NutGet Manager Solution

Step 1: Add AddNewtonsoftJson() in Startup.cs

                        public void ConfigureServices(IServiceCollection services)
                        {


                            services.AddControllers()
                                .AddNewtonsoftJson(ops =>
                                {
                                    ops.SerializerSettings.NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore;
                                });


                        }


                        

Step2: return json data

                        using Newtonsoft.Json;
                        .... 
                        .... 

                        Department d = new Department();
                        d.DepartmentId = 1;
                        d.DepartmentName = "m";
                        return JsonConvert.SerializeObject(d);