public class AppSettings
    {
        IConfiguration config;
        public AppSettings():this("appsettings.json") { }
        public AppSettings(string jsonFileName)
        {
            var configBuilder = new ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory()).AddJsonFile(jsonFileName);
            config = configBuilder.Build();
        }
        public string this[string name]
        {
            get { return config.GetSection(name).Value;}
        }
    }

用法

AppSettings get = new AppSettings();
string v = get["key"];