概述

我们都知道asp.net core配置信息的读取离不开IConfigurationSource和IConfigurationProvider这两个类，ConfigurationSource可以提供一个ConfigurationProvider，然后去读取信息。究竟他们之间有着怎样的千丝万缕，我们一起来看看源码。

首先我们来建立一个.net core控制台项目，来运行以下代码：

 class Program
    {
        static void Main(string[] args)
        {
            ConfigurationBuilder configBuilder = new ConfigurationBuilder();
            configBuilder.SetBasePath(Directory.GetCurrentDirectory())   
                   .AddJsonFile("appsettings.json");
            var configFile = configBuilder.Build();

            Console.ReadKey();
        }
    }

短短几行 代码看起来很简单，就是用来读取appsettings.json文件中的配置信息。然而我们今天想搞清楚其背后运行的原理，要花点时间。

首先、我们根据代码ConfigurationBuilder configBuilder = new ConfigurationBuilder();知道创建了一个configBuilder对象；

其次，configBuilder.SetBasePath(Directory.GetCurrentDirectory()) 该代码的调用我们也能大概见名知义，获取当前的目录；

接下来，重点来了，configBuilder.AddJsonFile("appsettings.json")的实现究竟是怎样的？我们来看下源码的实现：

f12进去后源码如下：

/// <summary>Extension methods for adding <see cref="T:Microsoft.Extensions.Configuration.Json.JsonConfigurationProvider" />.</summary>
  public static class JsonConfigurationExtensions
  {
    /// <summary>Adds the JSON configuration provider at <paramref name="path" /> to <paramref name="builder" />.</summary>
    /// <param name="builder">The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" /> to add to.</param>
    /// <param name="path">Path relative to the base path stored in
    /// <see cref="P:Microsoft.Extensions.Configuration.IConfigurationBuilder.Properties" /> of <paramref name="builder" />.</param>
    /// <returns>The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" />.</returns>
    public static IConfigurationBuilder AddJsonFile(
      this IConfigurationBuilder builder,
      string path)
    {
      return builder.AddJsonFile((IFileProvider) null, path, false, false);
    }
}

紧接着f12再看实现的源码，依然在JsonConfigurationExtensions这个扩展类里面：

    public static IConfigurationBuilder AddJsonFile(
      this IConfigurationBuilder builder,
      IFileProvider provider,
      string path,
      bool optional,
      bool reloadOnChange)
    {
      if (builder == null)
        throw new ArgumentNullException(nameof (builder));
      if (string.IsNullOrEmpty(path))
        throw new ArgumentException(SR.Error_InvalidFilePath, nameof (path));
      return builder.AddJsonFile((Action<JsonConfigurationSource>) (s =>
      {
        s.FileProvider = provider;
        s.Path = path;
        s.Optional = optional;
        s.ReloadOnChange = reloadOnChange;
        s.ResolveFileProvider();
      }));
    }

这时候有没有发现builder.AddJsonFile((Action)这个方法里面出现了一个关键的信息点：JsonConfigurationSource (JsonConfigurationSource 继承抽象类FileConfigurationSource,而FileConfigurationSource：IConfigurationSource) 。 关系如下图：

看完上面这个关系图后，我们紧接着上面builder.AddJsonFile()的实现源码继续f12往下,源码如下：

public static IConfigurationBuilder AddJsonFile(
    this IConfigurationBuilder builder,
    Action<JsonConfigurationSource> configureSource)
  {
    return ConfigurationExtensions.Add<JsonConfigurationSource>(builder, (Action<M0>) configureSource);
  }

我们看到上面的扩展方法实现是ConfigurationExtensions.Add...,再往下看实现：

public static class ConfigurationExtensions
  {
    /// <summary>Adds a new configuration source.</summary>
    /// <param name="builder">The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" /> to add to.</param>
    /// <param name="configureSource">Configures the source secrets.</param>
    /// <typeparam name="TSource" />
    /// <returns>The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" />.</returns>
    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);
    }
}

到这里我们看到了其实就是IConfigurationBuilder调用了Add方法，添加了一个数据源（JsonConfigurationSource），至于JsonConfigurationSource类里面做了什么，我们看下实现。

  public class JsonConfigurationSource : FileConfigurationSource
  {
    /// <summary>Builds the <see cref="T:Microsoft.Extensions.Configuration.Json.JsonConfigurationProvider" /> for this source.</summary>
    /// <param name="builder">The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" />.</param>
    /// <returns>A <see cref="T:Microsoft.Extensions.Configuration.Json.JsonConfigurationProvider" /></returns>
    public override IConfigurationProvider Build(IConfigurationBuilder builder)
    {
      this.EnsureDefaults(builder);
      return (IConfigurationProvider) new JsonConfigurationProvider(this);
    }
  }

JsonConfigurationSource类面的Build方法提供了一个JsonConfigurationProvider类，这里再贴下JsonConfigurationProvider类里面的代码：

  /// <summary>A JSON file based <see cref="T:Microsoft.Extensions.Configuration.FileConfigurationProvider" />.</summary>
  public class JsonConfigurationProvider : FileConfigurationProvider
  {
    /// <summary>Initializes a new instance with the specified source.</summary>
    /// <param name="source">The source settings.</param>
    public JsonConfigurationProvider(JsonConfigurationSource source)
      : base((FileConfigurationSource) source)
    {
    }

    /// <summary>Loads the JSON data from a stream.</summary>
    /// <param name="stream">The stream to read.</param>
    public virtual void Load(Stream stream)
    {
      try
      {
        this.set_Data(JsonConfigurationFileParser.Parse(stream));
      }
      catch (JsonException ex)
      {
        throw new FormatException(SR.Error_JSONParseError, (Exception) ex);
      }
    }
  }

关于JsonConfigurationProvider里面的Load就是去读取信息的实现，至于Load的具体实现我们不再深究。我们回到最初的控制台configBuilder.Build()，看看其的实现：

 {
   /// <summary>Returns the sources used to obtain configuration values.</summary>
   public IList<IConfigurationSource> Sources { get; } = (IList<IConfigurationSource>) new List<IConfigurationSource>();

   /// <summary>Gets a key/value collection that can be used to share data between the <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" />
   /// and the registered <see cref="T:Microsoft.Extensions.Configuration.IConfigurationProvider" />s.</summary>
   public IDictionary<string, object> Properties { get; } = (IDictionary<string, object>) new Dictionary<string, object>();

   /// <summary>Adds a new configuration source.</summary>
   /// <param name="source">The configuration source to add.</param>
   /// <returns>The same <see cref="T:Microsoft.Extensions.Configuration.IConfigurationBuilder" />.</returns>
   public IConfigurationBuilder Add(IConfigurationSource source)
   {
     if (source == null)
       throw new ArgumentNullException(nameof (source));
     this.Sources.Add(source);
     return (IConfigurationBuilder) this;
   }

   /// <summary>Builds an <see cref="T:Microsoft.Extensions.Configuration.IConfiguration" /> with keys and values from the set of providers registered in
   /// <see cref="P:Microsoft.Extensions.Configuration.ConfigurationBuilder.Sources" />.</summary>
   /// <returns>An <see cref="T:Microsoft.Extensions.Configuration.IConfigurationRoot" /> with keys and values from the registered providers.</returns>
   public IConfigurationRoot Build()
   {
     List<IConfigurationProvider> configurationProviderList = new List<IConfigurationProvider>();
     foreach (IConfigurationSource source in (IEnumerable<IConfigurationSource>) this.Sources)
     {
       IConfigurationProvider configurationProvider = source.Build((IConfigurationBuilder) this);
       configurationProviderList.Add(configurationProvider);
     }
     return (IConfigurationRoot) new ConfigurationRoot((IList<IConfigurationProvider>) configurationProviderList);
   }
 }

看到这个源码的时候有没有种豁然开朗的感觉，前面我们说到IConfigurationBuilder调用了Add方法添加一个数据源，并没说添加了一个数据源存在了哪里，到底有什么用处，现在在上面ConfigurationBuilder类里面看到存在了Sources 集合里面。然后configBuilder.Build()

去调用的时候遍历数据源（Sources ）集合，紧接着source (IConfigurationSource)调用了Build方法构建了一个configurationProvider对象存到configurationProviderList集合里面，最后在返回一个ConfigurationRoot对象的构造函数里面传递了configurationProviderList集合去执行。

贴上ConfigurationRoot的源码：

  {

    private readonly IList<IConfigurationProvider> _providers;
    private readonly IList<IDisposable> _changeTokenRegistrations;

    /// <summary>Initializes a Configuration root with a list of providers.</summary>
    /// <param name="providers">The <see cref="T:Microsoft.Extensions.Configuration.IConfigurationProvider" />s for this configuration.</param>
    public ConfigurationRoot(IList<IConfigurationProvider> providers)
    {
      if (providers == null)
        throw new ArgumentNullException(nameof (providers));
      this._providers = providers;
      this._changeTokenRegistrations = (IList<IDisposable>) new List<IDisposable>(providers.Count);
      foreach (IConfigurationProvider provider in (IEnumerable<IConfigurationProvider>) providers)
      {
        IConfigurationProvider p = provider;
        p.Load();
        this._changeTokenRegistrations.Add(ChangeToken.OnChange((Func<IChangeToken>) (() => p.GetReloadToken()), (Action) (() => this.RaiseChanged())));
      }
    }
}

看到没，最后providers去调用了load方法。

结语

就上面的控制台代码来说IConfigurationSource对应的实现是JsonConfigurationSource；IConfigurationProvider，抽象类ConfigurationProvider对应的实现为JsonConfigurationProvider。如果我们要换成别的文件格式呢？比如ini,怎样自定义配置源呢？大家可以先想想，其实也很简单，下次跟大家分享。

最后说真的，.netCore源码真的特别优秀，很值得花一番时间去看看！从其中可以学到许多架构知识！