.NET Core WebApi json data returned in the case of the first letter of the problem

void ConfigureServices public (IServiceCollection Services) 
{ 
    services.AddMvc () AddJsonOptions. (opt => 
    { 
       opt.SerializerSettings.ContractResolver new new Newtonsoft.Json.Serialization.DefaultContractResolver = (); // case of the string is output as JSON 
    }); 
}