如何使用Feature Toggle Nuget Package在.Net Core中启用功能切

using FeatureToggle;

namespace AspDotNetCoreExample.Models
{
    public class Printing : SimpleFeatureToggle {}
}

appSettings.json添加了featureToggle Key,并将其设置为true.因此,Printing类将读取它并启用功能切换.

{
  "FeatureToggle": {
    "Printing": "true"
  }
  }

** Startup.cs在ConfigureServices方法中注册了我的打印服务.我已将配置文件(appSettings.json)传递给Printing类.

public Startup(IHostingEnvironment env)
        {
            var builder = new ConfigurationBuilder()
                .SetBasePath(env.ContentRootPath)
                .AddJsonFile("appsettings.json",optional: false,reloadOnChange: true)
                .AddJsonFile($"appsettings.{env.EnvironmentName}.json",optional: true)
                .AddEnvironmentVariables();
            Configuration = builder.Build();
        }

    public IConfigurationRoot Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        // Set provider config so file is read from content root path
        var provider = new AppSettingsProvider { Configuration = Configuration };
        services.Configure<AppSettings(Configuration.GetSection("AppSettings"));
        services.AddSingleton(new Printing { ToggleValueProvider = provider });       
        services.AddTransient<IMapper,Mapper>();
        // Add framework services.
        services.AddMvc();
    }

//在Mapper类中,检查功能切换是否已启用并启用功能.

namespace STAR.Marketing.Portal.Web
{
    public class Mapper: Mapper
    {
        private readonly Printing _print;

        public Mapper(Printing print) //Dependency Injection to get Printing
        {
            _print = print;
        }
        public string Enabled()
        {
            return _print.FeatureEnabled;   // To check the feature is enabled but getting the Error System.FileIOException
        }
    }
}

我在上面的代码中犯了什么错误？为什么我在我的代码System.FileIOException中收到此错误.？