文件型配置的抽象扩展位于Microsoft.Extensions.Configuration.FileExtensions组件中，该扩展是一个基础实现。不过其命名空间是Microsoft.Extensions.Configuration，而Micros oft.Extensions.Configuration扩建本身又是整个.NET Core Configuration的基础实现。将File扩展独立于外部，体验了.NET Core的模块化设计。

FileConfigurationSource

Configuration.FileExtensions组件中，FileConfigurationSource是继承于IConfigurationSource的一个抽象类，包含了一个IConfigurationProvider类型的抽象方法，如下所示

   1:  /// <summary>

   2:  /// Builds the <see cref="IConfigurationProvider"/> for this source.

   3:  /// </summary>

   4:  /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>

   5:  /// <returns>A <see cref="IConfigurationProvider"/></returns>

   6:  public abstract IConfigurationProvider Build(IConfigurationBuilder builder);

该抽象类中还包括了几个比较重要的参数，分别用于配置性行为、文件内容访问以及异常处理。

string Path：文件的路径

bool Optional：标识加载的文件是否是可选的

bool ReloadOnChange：如果文件发生修改，是否重新加载配置源

int ReloadDelay：加载延迟，单位是毫秒，默认是250毫秒

IFileProvider FileProvider：用于获取文件内容

Action<FileLoadExceptionContext> OnLoadException：文件加载异常处理

该类对FileProvider有特殊处理，就是如果没有提供FileProvider实例，则会基于绝对路径，在最近的现有目录中创建物理文件提供程序。源码如下，

   1:  /// <summary>

   2:  /// If no file provider has been set, for absolute Path, this will creates a physical file provider

   3:  /// for the nearest existing directory.

   4:  /// </summary>

   5:  public void ResolveFileProvider()

   6:  {

   7:      if (FileProvider == null &&

   8:          !string.IsNullOrEmpty(Path) &&

   9:          System.IO.Path.IsPathRooted(Path))

  10:      {

11:

var directory = System.IO.Path.GetDirectoryName(Path);

  12:          var pathToFile = System.IO.Path.GetFileName(Path);

  13:          while (!string.IsNullOrEmpty(directory) && !Directory.Exists(directory))

  14:          {

  15:              pathToFile = System.IO.Path.Combine(System.IO.Path.GetFileName(directory), pathToFile);

  16:              directory = System.IO.Path.GetDirectoryName(directory);

  17:          }

  18:          if (Directory.Exists(directory))

  19:          {

  20:              FileProvider = new PhysicalFileProvider(directory);

  21:              Path = pathToFile;

  22:          }

  23:      }

  24:  }

FileConfigurationProvider

该类是继承于ConfigurationProvider的抽象类，是从文件系统加载配置的基类，同时还继承了IDisposable，其抽象方法是Load方法，用于从当前的Provider中以Stream方式加载数据

   1:  /// <summary>

   2:  /// Loads this provider's data from a stream.

   3:  /// </summary>

   4:  /// <param name="stream">The stream to read.</param>

   5:  public abstract void Load(Stream stream);

该类还重写了ConfigurationProvider的Load方法，并对文件加载中的异常做了处理，Data属性在前文有提到过，此处不再做其他说明。方法源码如下所示：

   1:  private void Load(bool reload)

   2:  {

   3:      var file = Source.FileProvider?.GetFileInfo(Source.Path);

   4:      if (file == null || !file.Exists)

   5:      {

   6:          if (Source.Optional || reload) // Always optional on reload

   7:          {

   8:              Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

   9:          }

  10:          else

  11:          {

  12:              var error = new StringBuilder($"The configuration file '{Source.Path}' was not found and is not optional.");

  13:              if (!string.IsNullOrEmpty(file?.PhysicalPath))

  14:              {

  15:                  error.Append($" The physical path is '{file.PhysicalPath}'.");

  16:              }

  17:              HandleException(new FileNotFoundException(error.ToString()));

  18:          }

  19:      }

  20:      else

  21:      {

  22:          // Always create new Data on reload to drop old keys

  23:          if (reload)

  24:          {

  25:              Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

  26:          }

  27:          using (var stream = file.CreateReadStream())

  28:          {

  29:              try

  30:              {

  31:                  Load(stream);

  32:              }

  33:              catch (Exception e)

  34:              {

  35:                  HandleException(e);

  36:              }

  37:          }

  38:      }

  39:      // REVIEW: Should we raise this in the base as well / instead?，通过注释，我们可以知道OnReload()方法可能会在新版中发生变化

  40:      OnReload();

  41:  }

42:

  43:  /// <summary>

  44:  /// Loads the contents of the file at <see cref="Path"/>.

  45:  /// </summary>

  46:  /// <exception cref="FileNotFoundException">If Optional is <c>false</c> on the source and a

  47:  /// file does not exist at specified Path.</exception>

  48:  public override void Load()

  49:  {

  50:      Load(reload: false);

  51:  }

另外它还有一个特殊方法，就是参数类型为FileConfigurationSource的构造函数，其主要功能是监控文件，并在FileConfigurationSource.ReloadDelay设置的时间里重新加载文件并返回一个IDisposable类型的值，以下是该构造函数的源码：

   1:  /// <summary>

   2:  /// Initializes a new instance with the specified source.

   3:  /// </summary>

   4:  /// <param name="source">The source settings.</param>

   5:  public FileConfigurationProvider(FileConfigurationSource source)

   6:  {

   7:      if (source == null)

   8:      {

   9:          throw new ArgumentNullException(nameof(source));

  10:      }

  11:      Source = source;

12:

  13:      if (Source.ReloadOnChange && Source.FileProvider != null)

  14:      {

  15:          _changeTokenRegistration = ChangeToken.OnChange(

  16:              () => Source.FileProvider.Watch(Source.Path),

  17:              () => {

  18:                  Thread.Sleep(Source.ReloadDelay);

  19:                  Load(reload: true);

  20:              });

  21:      }

  22:  }

FileConfigurationExtensions

该类是一个静态类，其提供了的多个扩展方法，主要基于

IConfigurationBuilder
IFileProvider
Action<FileLoadExceptionContext>

包括主要用于设置或获取IFileProvider对象，前文有介绍过，是存储于字典之中，需要注意的是，在Get的时候如果字典中并不存在IFileProvider对象，则会实例化一个PhysicalFileProvider对象出来，该类位于Microsoft.Extensions.FileProviders.PhysicalFileProvider

   1:  /// <summary>

   2:  /// Sets the default <see cref="IFileProvider"/> to be used for file-based providers.

   3:  /// </summary>

   4:  /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param>

   5:  /// <param name="fileProvider">The default file provider instance.</param>

   6:  /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>

   7:  public static IConfigurationBuilder SetFileProvider(this IConfigurationBuilder builder, IFileProvider fileProvider)

   8:  {

   9:      if (builder == null)

  10:      {

  11:          throw new ArgumentNullException(nameof(builder));

  12:      }

13:

  14:      builder.Properties[FileProviderKey] = fileProvider ?? throw new ArgumentNullException(nameof(fileProvider));

  15:      return builder;

  16:  }

17:

  18:  /// <summary>

  19:  /// Gets the default <see cref="IFileProvider"/> to be used for file-based providers.

  20:  /// </summary>

  21:  /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>

  22:  /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>

  23:  public static IFileProvider GetFileProvider(this IConfigurationBuilder builder)

  24:  {

  25:      if (builder == null)

  26:      {

  27:          throw new ArgumentNullException(nameof(builder));

  28:      }

29:

  30:      if (builder.Properties.TryGetValue(FileProviderKey, out object provider))

  31:      {

  32:          return provider as IFileProvider;

  33:      }

34:

  35:      return new PhysicalFileProvider(AppContext.BaseDirectory ?? string.Empty);

  36:  }

为指定路径的物理文件设置文件型Provider，该方法同样基于PhysicalFileProvider，并返回IConfigurationBuilder对象

   1:  /// <summary>

   2:  /// Sets the FileProvider for file-based providers to a PhysicalFileProvider with the base path.

   3:  /// </summary>

   4:  /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param>

   5:  /// <param name="basePath">The absolute path of file-based providers.</param>

   6:  /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>

   7:  public static IConfigurationBuilder SetBasePath(this IConfigurationBuilder builder, string basePath)

   8:  {

   9:      if (builder == null)

  10:      {

  11:          throw new ArgumentNullException(nameof(builder));

  12:      }

13:

  14:      if (basePath == null)

  15:      {

  16:          throw new ArgumentNullException(nameof(basePath));

  17:      }

18:

  19:      return builder.SetFileProvider(new PhysicalFileProvider(basePath));

  20:  }

以及异常处理，可以看到其异常处理也会存放于字典中，如果字典中找不到，就会返回空，这个地方如果直接使用，需要注意空指针问题。

   1:  /// <summary>

   2:  /// Sets a default action to be invoked for file-based providers when an error occurs.

   3:  /// </summary>

   4:  /// <param name="builder">The <see cref="IConfigurationBuilder"/> to add to.</param>

   5:  /// <param name="handler">The Action to be invoked on a file load exception.</param>

   6:  /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>

   7:  public static IConfigurationBuilder SetFileLoadExceptionHandler(this IConfigurationBuilder builder, Action<FileLoadExceptionContext> handler)

   8:  {

   9:      if (builder == null)

  10:      {

  11:          throw new ArgumentNullException(nameof(builder));

  12:      }

13:

  14:      builder.Properties[FileLoadExceptionHandlerKey] = handler;

  15:      return builder;

  16:  }

17:

  18:  /// <summary>

  19:  /// Gets the default <see cref="IFileProvider"/> to be used for file-based providers.

  20:  /// </summary>

  21:  /// <param name="builder">The <see cref="IConfigurationBuilder"/>.</param>

  22:  /// <returns>The <see cref="IConfigurationBuilder"/>.</returns>

  23:  public static Action<FileLoadExceptionContext> GetFileLoadExceptionHandler(this IConfigurationBuilder builder)

  24:  {

  25:      if (builder == null)

  26:      {

  27:          throw new ArgumentNullException(nameof(builder));

  28:      }

29:

  30:      if (builder.Properties.TryGetValue(FileLoadExceptionHandlerKey, out object handler))

  31:      {

  32:          return handler as Action<FileLoadExceptionContext>;

  33:      }

34:

return null;

  35:  }

该类还有两个静态私有变量，指定了文件Provider的Key以及文件加载异常处理Key。

   1:  private static string FileProviderKey = "FileProvider";

   2:  private static string FileLoadExceptionHandlerKey = "FileLoadExceptionHandler";

总结

文件型配置还依赖于.NET Core的其他组件Microsoft.Extensions.FileProviders和Microsoft.Extensions.Primitives。

FileProviders组件提供了文件处理的一般方法，Primitives组件提供了监控机制，同时还包括两个比较重要的结构体StringValues和StringSegment，本文暂时不做讨论，有兴趣的朋友，可以自行查看该组件源码。

.NET Core 3.0之深入源码理解Configuration

.NET Core 3.0之深入源码理解Configuration(二)

文件型配置基本内容

文件型配置的抽象扩展

FileConfigurationSource

FileConfigurationProvider

FileConfigurationExtensions

总结

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