public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
  WebHost.CreateDefaultBuilder(args)
  .ConfigureAppConfiguration(option =>
   {
   option.AddJsonFile("appsettings.json",optional:true,reloadOnChange:true);
   })
  .UseStartup<Startup>();

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder,string path,bool optional,bool reloadOnChange)
 {
 return builder.AddJsonFile((IFileProvider) null, path, optional, reloadOnChange);
 }

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder,IFileProvider provider,string path,bool optional,bool reloadOnChange)
 {
  return builder.AddJsonFile((Action<JsonConfigurationSource>) (s =>
  {
  s.FileProvider = provider;
  s.Path = path;
  s.Optional = optional;
  s.ReloadOnChange = reloadOnChange;
  s.ResolveFileProvider();
  }));
 }

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder, Action<JsonConfigurationSource> configureSource)
 {
  return builder.Add<JsonConfigurationSource>(configureSource);
 }

 public static IConfigurationBuilder Add<TSource>(this IConfigurationBuilder builder,Action<TSource> configureSource)where TSource : IConfigurationSource, new()
 {
  TSource source = new TSource();
  if (configureSource != null)
  configureSource(source);
  return builder.Add((IConfigurationSource) source);
 }

public class ConfigurationBuilder : IConfigurationBuilder
 {
  public IList<IConfigurationSource> Sources { get; } = new List<IConfigurationSource>();

  public IConfigurationBuilder Add(IConfigurationSource source)
  {
   Sources.Add(source);
   return this;
  }

  public IConfigurationRoot Build()
  {
   var providers = new List<IConfigurationProvider>();
   foreach (var source in Sources)
   {
    var provider = source.Build(this);
    providers.Add(provider);
   }
   return new ConfigurationRoot(providers);
  }
 }

public static void Main(string[] args)
  {
   CreateWebHostBuilder(args).Build().Run();
  }

public class JsonConfigurationSource : FileConfigurationSource
 {
  public override IConfigurationProvider Build(IConfigurationBuilder builder)
  {
   EnsureDefaults(builder);
   return new JsonConfigurationProvider(this);
  }
 }

 public void EnsureDefaults(IConfigurationBuilder builder)
  {
   FileProvider = FileProvider ?? builder.GetFileProvider();
  }
  public static IFileProvider GetFileProvider(this IConfigurationBuilder builder)
  {
   return new PhysicalFileProvider(AppContext.BaseDirectory ?? string.Empty);
  }

 public class JsonConfigurationProvider : FileConfigurationProvider
 {
  
  public JsonConfigurationProvider(JsonConfigurationSource source) : base(source) { }

  
  public override void Load(Stream stream)
  {
   try {
    Data = JsonConfigurationFileParser.Parse(stream);
   } catch (JsonReaderException e)
   {
    throw new FormatException(Resources.Error_JSONParseError, e);
   }
  }
 }

 public FileConfigurationProvider(FileConfigurationSource source)
  {
   Source = source;

   if (Source.ReloadOnChange && Source.FileProvider != null)
   {
    _changeTokenRegistration = ChangeToken.OnChange(
     () => Source.FileProvider.Watch(Source.Path),
     () => {
      Thread.Sleep(Source.ReloadDelay);
      Load(reload: true);
     });
   }
  }

 public static class ChangeToken
 {
  public static ChangeTokenRegistration<Action> OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
  {
   return new ChangeTokenRegistration<Action>(changeTokenProducer, callback => callback(), changeTokenConsumer);
  }
 }

() => Source.FileProvider.Watch(Source.Path)

() => {
 Thread.Sleep(Source.ReloadDelay);
 Load(reload: true);
}

 public interface IChangeToken
 {
  bool HasChanged { get; }

  bool ActiveChangeCallbacks { get; }

  IDisposable RegisterChangeCallback(Action<object> callback, object state);
 }

 static void Main()
  {
   //定义一个C:\Users\liuzh\MyBox\TestSpace目录的FileProvider
   var phyFileProvider = new PhysicalFileProvider("C:\\Users\\liuzh\\MyBox\\TestSpace");

   //让这个Provider开始监听这个目录下的所有文件
   var changeToken = phyFileProvider.Watch("*.*");

   //注册🐍吃🐀这件事到回调函数
   changeToken.RegisterChangeCallback(_=> { Console.WriteLine("老鼠被蛇吃"); }, new object());

   //添加一个文件到目录
   AddFileToPath();

   Console.ReadKey();

  }

  static void AddFileToPath()
  {
   Console.WriteLine("老鼠出洞了");
   File.Create("C:\\Users\\liuzh\\MyBox\\TestSpace\\老鼠出洞了.txt").Dispose();
  }

 public static class ChangeToken
 {
  public static ChangeTokenRegistration<Action> OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
  {
   return new ChangeTokenRegistration<Action>(changeTokenProducer, callback => callback(), changeTokenConsumer);
  }
 }
 public class ChangeTokenRegistration<TAction>
 {
  private readonly Func<IChangeToken> _changeTokenProducer;
  private readonly Action<TAction> _changeTokenConsumer;
  private readonly TAction _state;

  public ChangeTokenRegistration(Func<IChangeToken> changeTokenProducer, Action<TAction> changeTokenConsumer, TAction state)
  {
   _changeTokenProducer = changeTokenProducer;
   _changeTokenConsumer = changeTokenConsumer;
   _state = state;

   var token = changeTokenProducer();

   RegisterChangeTokenCallback(token);
  }

  private void RegisterChangeTokenCallback(IChangeToken token)
  {
   token.RegisterChangeCallback(_ => OnChangeTokenFired(), this);
  }

  private void OnChangeTokenFired()
  {
   var token = _changeTokenProducer();

   try
   {
    _changeTokenConsumer(_state);
   }
   finally
   {
    // We always want to ensure the callback is registered
    RegisterChangeTokenCallback(token);
   }
  }
 }

  static void Main()
  {
   var phyFileProvider = new PhysicalFileProvider("C:\\Users\\liuzh\\MyBox\\TestSpace");
   ChangeToken.OnChange(() => phyFileProvider.Watch("*.*"),
    () => { Console.WriteLine("老鼠被蛇吃"); });
   Console.ReadKey();
  }

 public PhysicalFilesWatcher(string root,FileSystemWatcher fileSystemWatcher,bool pollForChanges,ExclusionFilters filters)
 {
  this._root = root;
  this._fileWatcher = fileSystemWatcher;
  this._fileWatcher.IncludeSubdirectories = true;
  this._fileWatcher.Created += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Changed += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Renamed += new RenamedEventHandler(this.OnRenamed);
  this._fileWatcher.Deleted += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Error += new ErrorEventHandler(this.OnError);
  this.PollForChanges = pollForChanges;
  this._filters = filters;
  this.PollingChangeTokens = new ConcurrentDictionary<IPollingChangeToken, IPollingChangeToken>();
  this._timerFactory = (Func<Timer>) (() => NonCapturingTimer.Create(new TimerCallback(PhysicalFilesWatcher.RaiseChangeEvents), (object) this.PollingChangeTokens, TimeSpan.Zero, PhysicalFilesWatcher.DefaultPollingInterval));
 }