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！
