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()
 {
 //定義一個C:\Users\liuzh\MyBox\TestSpace目錄的FileProvider
 var phyFileProvider = new PhysicalFileProvider("C:\\Users\\liuzh\\MyBox\\TestSpace");

 //讓這個Provider開始監聽這個目錄下的所有文件
 var changeToken = phyFileProvider.Watch("*.*");

 //注冊🐍吃🐀這件事到回調函數
 changeToken.RegisterChangeCallback(_=> { Console.WriteLine("老鼠被蛇吃"); }, new object());

 //添加一個文件到目錄
 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));
 }