概述
我們都知道asp.net core配置資訊的讀取離不開IConfigurationSource和IConfigurationProvider這兩個類，ConfigurationSource可以提供一個ConfigurationProvider，然後去讀取資訊。究竟他們之間有著怎樣的千絲萬縷，我們一起來看看原始碼。
首先我們來建立一個.net core控制檯專案，來執行以下程式碼：
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 程式碼看起來很簡單，就是用來讀取appsettings.json檔案中的配置資訊。然而我們今天想搞清楚其背後執行的原理，要花點時間。
首先、我們根據程式碼ConfigurationBuilder configBuilder = new ConfigurationBuilder();知道建立了一個configBuilder物件；
其次，configBuilder.SetBasePath(Directory.GetCurrentDirectory()) 該程式碼的呼叫我們也能大概見名知義，獲取當前的目錄；
接下來，重點來了，configBuilder.AddJsonFile("appsettings.json")的實現究竟是怎樣的？我們來看下原始碼的實現：
f12進去後原始碼如下：
/// <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); } }
緊接著f12再看實現的原始碼，依然在JsonConfigurationExtensions這個擴充套件類裡面：
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(); })); }
這時候有沒有發現builder.AddJsonFile((Action<JsonConfigurationSource>)這個方法裡面出現了一個關鍵的資訊點：JsonConfigurationSource (JsonConfigurationSource 繼承抽象類FileConfigurationSource,而FileConfigurationSource：IConfigurationSource) 。 關係如下圖：
看完上面這個關係圖後，我們緊接著上面builder.AddJsonFile()的實現原始碼繼續f12往下,原始碼如下：
//該程式碼依然在JsonConfigurationExtensions類裡面
public static IConfigurationBuilder AddJsonFile( this IConfigurationBuilder builder, Action<JsonConfigurationSource> configureSource) { return ConfigurationExtensions.Add<JsonConfigurationSource>(builder, (Action<M0>) configureSource); }
我們看到上面的擴充套件方法實現是ConfigurationExtensions.Add...,再往下看實現：
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); } }
到這裡我們看到了其實就是IConfigurationBuilder呼叫了Add方法，新增了一個資料來源（JsonConfigurationSource），至於JsonConfigurationSource類裡面做了什麼，我們看下實現。
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類面的Build方法提供了一個JsonConfigurationProvider類，這裡再貼下JsonConfigurationProvider類裡面的程式碼：
/// <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); } } }
關於JsonConfigurationProvider裡面的Load就是去讀取資訊的實現，至於Load的具體實現我們不再深究。我們回到最初的控制檯configBuilder.Build()，看看其的實現：
public class ConfigurationBuilder : IConfigurationBuilder { /// <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); } }
看到這個原始碼的時候有沒有種豁然開朗的感覺，前面我們說到IConfigurationBuilder呼叫了Add方法新增一個資料來源，並沒說新增了一個資料來源存在了哪裡，到底有什麼用處，現在在上面ConfigurationBuilder類裡面看到存在了Sources 集合裡面。然後configBuilder.Build()
去呼叫的時候遍歷資料來源（Sources ）集合，緊接著source (IConfigurationSource)呼叫了Build方法構建了一個configurationProvider物件存到configurationProviderList集合裡面，最後在返回一個ConfigurationRoot物件的建構函式裡面傳遞了configurationProviderList集合去執行。
貼上ConfigurationRoot的原始碼：
public class ConfigurationRoot : IConfigurationRoot, IConfiguration, IDisposable { 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()))); } } }
看到沒，最後providers去呼叫了load方法。
結語
就上面的控制檯程式碼來說IConfigurationSource對應的實現是JsonConfigurationSource；IConfigurationProvider，抽象類ConfigurationProvider對應的實現為JsonConfigurationProvider。如果我們要換成別的檔案格式呢？比如ini,怎樣自定義配置源呢？大家可以先想想，其實也很簡單，下次跟大家分享。
最後說真的，.netCore原始碼真的特別優秀，很值得花一番時間去看看！從其中可以學到許多架構知識！