.NET Core 選項系統的主要實現在 Microsoft.Extensions.Options 和 Microsoft.Extensions.Options.ConfigurationExtensions 兩個 Nuget 包。對於一個框架的源碼進行解讀，我們可以從我們常用的框架中的類或方法入手 ...

.NET Core 選項系統的主要實現在 Microsoft.Extensions.Options 和 Microsoft.Extensions.Options.ConfigurationExtensions 兩個 Nuget 包。對於一個框架的源碼進行解讀，我們可以從我們常用的框架中的類或方法入手，這些類或方法就是我們解讀的入口。

從上面對選項系統的介紹中，大家也可以看出，日常對選項系統的使用涉及到的主要有 Configure 方法，有 IOptions、IOptionsSnapshot、IOptionMonitor 等介面。

Configure

首先看選項註冊，也就是 Configure 方法，註冊相關的方法都是擴展方法，上面也講到 Configure 方法有多個擴展來源，其中最常用的是 OptionsConfigurationServiceCollectionExtensions 中的 Configure 方法，該方法用於從配置信息中讀取配置並綁定為選項，如下，這裡將相應的方法單獨摘出來了。

點擊查看代碼 OptionsConfigurationServiceCollectionExtensions.Configure

public static class OptionsConfigurationServiceCollectionExtensions
{
	/// <summary>
	/// Registers a configuration instance which TOptions will bind against.
	/// </summary>
	/// <typeparam name="TOptions">The type of options being configured.</typeparam>
	/// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
	/// <param name="config">The configuration being bound.</param>
	/// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public static IServiceCollection Configure<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] TOptions>(this IServiceCollection services, IConfiguration config) where TOptions : class
		=> services.Configure<TOptions>(Options.Options.DefaultName, config);

	/// <summary>
	/// Registers a configuration instance which TOptions will bind against.
	/// </summary>
	/// <typeparam name="TOptions">The type of options being configured.</typeparam>
	/// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
	/// <param name="name">The name of the options instance.</param>
	/// <param name="config">The configuration being bound.</param>
	/// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public static IServiceCollection Configure<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] TOptions>(this IServiceCollection services, string name, IConfiguration config) where TOptions : class
		=> services.Configure<TOptions>(name, config, _ => { });

	/// <summary>
	/// Registers a configuration instance which TOptions will bind against.
	/// </summary>
	/// <typeparam name="TOptions">The type of options being configured.</typeparam>
	/// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
	/// <param name="config">The configuration being bound.</param>
	/// <param name="configureBinder">Used to configure the <see cref="BinderOptions"/>.</param>
	/// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public static IServiceCollection Configure<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] TOptions>(this IServiceCollection services, IConfiguration config, Action<BinderOptions> configureBinder)
		where TOptions : class
		=> services.Configure<TOptions>(Options.Options.DefaultName, config, configureBinder);

	/// <summary>
	/// Registers a configuration instance which TOptions will bind against.
	/// </summary>
	/// <typeparam name="TOptions">The type of options being configured.</typeparam>
	/// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
	/// <param name="name">The name of the options instance.</param>
	/// <param name="config">The configuration being bound.</param>
	/// <param name="configureBinder">Used to configure the <see cref="BinderOptions"/>.</param>
	/// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public static IServiceCollection Configure<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] TOptions>(this IServiceCollection services, string name, IConfiguration config, Action<BinderOptions> configureBinder)
		where TOptions : class
	{
		if (services == null)
		{
			throw new ArgumentNullException(nameof(services));
		}

		if (config == null)
		{
			throw new ArgumentNullException(nameof(config));
		}

		services.AddOptions();
		services.AddSingleton<IOptionsChangeTokenSource<TOptions>>(new ConfigurationChangeTokenSource<TOptions>(name, config));
		return services.AddSingleton<IConfigureOptions<TOptions>>(new NamedConfigureFromConfigurationOptions<TOptions>(name, config, configureBinder));
	}
}

其中 IOptionsChangeTokenSource 介面是用來監聽配置變化的服務，這個後面講。

另外還有 OptionsServiceCollectionExtensions 中的 Configure 方法，用於直接通過委托對選項類進行配置。

點擊查看代碼 OptionsServiceCollectionExtensions.Configure

public static class OptionsServiceCollectionExtensions
{

	public static IServiceCollection Configure<TOptions>(this IServiceCollection services, Action<TOptions> configureOptions) where TOptions : class
		=> services.Configure(Options.Options.DefaultName, configureOptions);

	public static IServiceCollection Configure<TOptions>(this IServiceCollection services, string name, Action<TOptions> configureOptions)
		where TOptions : class
	{
		if (services == null)
		{
			throw new ArgumentNullException(nameof(services));
		}

		if (configureOptions == null)
		{
			throw new ArgumentNullException(nameof(configureOptions));
		}

		services.AddOptions();
		services.AddSingleton<IConfigureOptions<TOptions>>(new ConfigureNamedOptions<TOptions>(name, configureOptions));
		return services;
	}
}

可以看出，其實選項系統中的選項都是命名模式的，預設名稱為 Options.Options.DefaultName，實際就是 string.Empty。當我們調用 Configure 方法對選項進行配置的時候，實際上時調用了 AddOptions 方法，並且往容器中添加了一個單例的實現了 IConfigureOptions 介面的實現。

IConfigureOptions、IConfigureNamedOptions、IPostConfigureOptions

其中 IConfigureOptions 是選項配置行為服務介面，ConfigureOptions 是它的預設實現，該類的內容很簡單，它的內部主要就是保存了一個委托，用於記錄使用者對選項的配置操作。

點擊查看代碼 ConfigureOptions

public class ConfigureOptions<TOptions> : IConfigureOptions<TOptions> where TOptions : class
{
	/// <summary>
	/// Constructor.
	/// </summary>
	/// <param name="action">The action to register.</param>
	public ConfigureOptions(Action<TOptions> action)
	{
		Action = action;
	}

	/// <summary>
	/// The configuration action.
	/// </summary>
	public Action<TOptions> Action { get; }

	/// <summary>
	/// Invokes the registered configure <see cref="Action"/>.
	/// </summary>
	/// <param name="options">The options instance to configure.</param>
	public virtual void Configure(TOptions options)
	{
		if (options == null)
		{
			throw new ArgumentNullException(nameof(options));
		}

		Action?.Invoke(options);
	}
}

IConfigureNamedOptions 繼承了 IConfigureNamedOptions 介面，預設實現是 ConfigureNamedOptions ，作用一樣，只不過多了一個方法用於應對命名選項模式。它有多個重載泛型重載，也是之前的文章ASP.NET Core - 選型系統之選型配置中講到的“使用DI服務配置選項”的具體實現。

點擊查看代碼 ConfigureNamedOptions

```csharp public class ConfigureNamedOptions : IConfigureNamedOptions where TOptions : class { ///

/// Constructor. ///

/// The name of the options. /// The action to register. public ConfigureNamedOptions(string name, Action action) { Name = name; Action = action; }

/// <summary>
/// The options name.
/// </summary>
public string Name { get; }

/// <summary>
/// The configuration action.
/// </summary>
public Action<TOptions> Action { get; }

/// <summary>
/// Invokes the registered configure <see cref="Action"/> if the <paramref name="name"/> matches.
/// </summary>
/// <param name="name">The name of the options instance being configured.</param>
/// <param name="options">The options instance to configure.</param>
public virtual void Configure(string name, TOptions options)
{
	if (options == null)
	{
		throw new ArgumentNullException(nameof(options));
	}

	// Null name is used to configure all named options.
	if (Name == null || name == Name)
	{
		Action?.Invoke(options);
	}
}

/// <summary>
/// Invoked to configure a <typeparamref name="TOptions"/> instance with the <see cref="Options.DefaultName"/>.
/// </summary>
/// <param name="options">The options instance to configure.</param>
public void Configure(TOptions options) => Configure(Options.DefaultName, options);

}

</details>

而 NamedConfigureFromConfigurationOptions<TOptions> 類是 IConfigureNamedOptions<TOptions> 的另一個實現，繼承了ConfigureNamedOptions<TOptions> 類，重寫了一些行為，最終是通過之前講到的 ConfigurationBuilder的 Bind 方法將配置綁定到選項類而已。

<details>
<summary>點擊查看代碼 NamedConfigureFromConfigurationOptions<TOptions></summary>

```csharp
public class NamedConfigureFromConfigurationOptions<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] TOptions> : ConfigureNamedOptions<TOptions>
	where TOptions : class
{
	/// <summary>
	/// Constructor that takes the <see cref="IConfiguration"/> instance to bind against.
	/// </summary>
	/// <param name="name">The name of the options instance.</param>
	/// <param name="config">The <see cref="IConfiguration"/> instance.</param>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public NamedConfigureFromConfigurationOptions(string name, IConfiguration config)
		: this(name, config, _ => { })
	{ }

	/// <summary>
	/// Constructor that takes the <see cref="IConfiguration"/> instance to bind against.
	/// </summary>
	/// <param name="name">The name of the options instance.</param>
	/// <param name="config">The <see cref="IConfiguration"/> instance.</param>
	/// <param name="configureBinder">Used to configure the <see cref="BinderOptions"/>.</param>
	[RequiresUnreferencedCode(OptionsBuilderConfigurationExtensions.TrimmingRequiredUnreferencedCodeMessage)]
	public NamedConfigureFromConfigurationOptions(string name, IConfiguration config, Action<BinderOptions> configureBinder)
		: base(name, options => BindFromOptions(options, config, configureBinder))
	{
		if (config == null)
		{
			throw new ArgumentNullException(nameof(config));
		}
	}

	[UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
		Justification = "The only call to this method is the constructor which is already annotated as RequiresUnreferencedCode.")]
	private static void BindFromOptions(TOptions options, IConfiguration config, Action<BinderOptions> configureBinder) => config.Bind(options, configureBinder);
}

其他的 IPostConfigureOptions 介面也是一樣套路，當我們通過相應的方法傳入委托對選項類進行配置的時候，會向容器中註入一個單例服務，將配置行為保存起來。

接著往下看 AddOptions 方法，AddOptions 方法有兩個重載：

點擊查看代碼 AddOptions

```csharp public static class OptionsServiceCollectionExtensions { public static IServiceCollection AddOptions(this IServiceCollection services) { if (services == null) { throw new ArgumentNullException(nameof(services)); }

	services.TryAdd(ServiceDescriptor.Singleton(typeof(IOptions<>), typeof(UnnamedOptionsManager<>)));
	services.TryAdd(ServiceDescriptor.Scoped(typeof(IOptionsSnapshot<>), typeof(OptionsManager<>)));
	services.TryAdd(ServiceDescriptor.Singleton(typeof(IOptionsMonitor<>), typeof(OptionsMonitor<>)));
	services.TryAdd(ServiceDescriptor.Transient(typeof(IOptionsFactory<>), typeof(OptionsFactory<>)));
	services.TryAdd(ServiceDescriptor.Singleton(typeof(IOptionsMonitorCache<>), typeof(OptionsCache<>)));
	return services;
}
public static OptionsBuilder<TOptions> AddOptions<TOptions>(this IServiceCollection services, string name)
	where TOptions : class
{
	if (services == null)
	{
		throw new ArgumentNullException(nameof(services));
	}

	services.AddOptions();
	return new OptionsBuilder<TOptions>(services, name);
}

}

</details>

這裡可以看出兩者的返回值不同，而且第二個方法也調用了第一個方法，第一個方法中主要就是向容器中添加我們常用的IOptions<TOptions>、IOptionsSnapshot<TOptions>、IOptionsMonitor<TOptions> 服務介面，這裡也可以看到不同服務介面對於的生命周期。除此之外還有工廠服務IOptionsFactory<>和緩存服務IOptionsMonitorCache<>，這兩個就是選項體系的關鍵。每個選項進行配置的時候都會同時註入這些服務，所以每一個選項我們都能使用三個不同介面去解析。

# OptionsBuilder

上面第二個 AddOptions 方法返回 OptionsBuilder<TOptions> 對象。之前講過 OptionsBuilder<TOptions> 類中也有 Configure 方法，其實不止 Configure 方法，其他的 PostConfigure 方法等也有，它其實就是最終的選項系統配置類，我們所有的選項配置其實都可以通過調用第二個 AddOptions 方法，再通過 OptionsBuilder<TOptions> 對象中的方法來完成配置。其他各個擴展方法的配置方式不過是進行了使用簡化而已。

<details>
<summary>點擊查看代碼 OptionsBuilder<TOptions></summary>

```csharp
public class OptionsBuilder<TOptions> where TOptions : class
{
	private const string DefaultValidationFailureMessage = "A validation error has occurred.";
	
	public string Name { get; }
	
	public IServiceCollection Services { get; }
	
	public OptionsBuilder(IServiceCollection services, string name)
	{
		Services = services;
		Name = name ?? Options.DefaultName;
	}
	
	public virtual OptionsBuilder<TOptions> Configure(Action<TOptions> configureOptions)
	{
		Services.AddSingleton<IConfigureOptions<TOptions>>(new ConfigureNamedOptions<TOptions>(Name, configureOptions));
		return this;
	}
	
	public virtual OptionsBuilder<TOptions> PostConfigure(Action<TOptions> configureOptions)
	{
		Services.AddSingleton<IPostConfigureOptions<TOptions>>(new PostConfigureOptions<TOptions>(Name, configureOptions));
		return this;
	}
	
	public virtual OptionsBuilder<TOptions> Validate(Func<TOptions, bool> validation)
		=> Validate(validation: validation, failureMessage: DefaultValidationFailureMessage);
		
	public virtual OptionsBuilder<TOptions> Validate(Func<TOptions, bool> validation, string failureMessage)
	{
		Services.AddSingleton<IValidateOptions<TOptions>>(new ValidateOptions<TOptions>(Name, validation, failureMessage));
		return this;
	}
}

IValidateOptions

我們除了可以對選項進行配置綁定之外，還可以對選項進行驗證。驗證規則是通過上面的第二個 AddOptions 方法返回的 OptionsBuilder 方法進行添加的。

驗證規則配置有三種方式，最後其實都是通過 IValidateOptions 的實現類來完成。我們自己實現的自定義驗證類就不用說了，最後我們會將其註入到容器中，而從上面的代碼中可以看到，當我們通過委托的方式自定義驗證規則的時候，它會被構建成一個 ValidateOptions 類對象，並註入到容器中作為一個服務。

ValidateOptions 是 IValidateOptions 的一個實現類，構造函數中接收委托，通過委托返回的 bool 結構判斷驗證是否通過。

點擊查看代碼 ValidateOptions

public class ValidateOptions<TOptions> : IValidateOptions<TOptions> where TOptions : class
{
	/// <summary>
	/// Constructor.
	/// </summary>
	/// <param name="name">Options name.</param>
	/// <param name="validation">Validation function.</param>
	/// <param name="failureMessage">Validation failure message.</param>
	public ValidateOptions(string name, Func<TOptions, bool> validation, string failureMessage)
	{
		Name = name;
		Validation = validation;
		FailureMessage = failureMessage;
	}

	/// <summary>
	/// The options name.
	/// </summary>
	public string Name { get; }

	/// <summary>
	/// The validation function.
	/// </summary>
	public Func<TOptions, bool> Validation { get; }

	/// <summary>
	/// The error to return when validation fails.
	/// </summary>
	public string FailureMessage { get; }

	/// <summary>
	/// Validates a specific named options instance (or all when <paramref name="name"/> is null).
	/// </summary>
	/// <param name="name">The name of the options instance being validated.</param>
	/// <param name="options">The options instance.</param>
	/// <returns>The <see cref="ValidateOptionsResult"/> result.</returns>
	public ValidateOptionsResult Validate(string name, TOptions options)
	{
		// null name is used to configure all named options
		if (Name == null || name == Name)
		{
			if ((Validation?.Invoke(options)).Value)
			{
				return ValidateOptionsResult.Success;
			}
			return ValidateOptionsResult.Fail(FailureMessage);
		}

		// ignored if not validating this instance
		return ValidateOptionsResult.Skip;
	}
}

我們可以通過重載方法傳入相應的驗證失敗提醒文本。

Options、UnnamedOptionsManager

接下來看選項使用相關的內容，其中 IOptions 中的選項類一經創建一直保持不變，預設實現類 UnnamedOptionsManager。

點擊查看代碼 UnnamedOptionsManager

internal sealed class UnnamedOptionsManager<[DynamicallyAccessedMembers(Options.DynamicallyAccessedMembers)] TOptions> :
	IOptions<TOptions>
	where TOptions : class
{
	private readonly IOptionsFactory<TOptions> _factory;
	private volatile object _syncObj;
	private volatile TOptions _value;

	public UnnamedOptionsManager(IOptionsFactory<TOptions> factory) => _factory = factory;

	public TOptions Value
	{
		get
		{
			if (_value is TOptions value)
			{
				return value;
			}

			lock (_syncObj ?? Interlocked.CompareExchange(ref _syncObj, new object(), null) ?? _syncObj)
			{
				return _value ??= _factory.Create(Options.DefaultName);
			}
		}
	}
}

IOptions 介面只有一個 Value 屬性，實現類中通過鎖確保創建的 Value 值不會因為線程問題導致不同，且該服務被註冊為單例生命周期，所以對象不銷毀，後續一直會讀取記憶體中的 Value 值。具體選項類對象的創建由工廠服務負責。

IOptionsSnapshot、OptionsManager

IOptionsSnapshot 的實現類是 OptionsManager，該類中有一個私有的 OptionsCache 屬性，每次對選項類進行讀取的時候，都是先嘗試從緩存讀取，如果沒有才創建。而由於 IOptionsSnapshot 被註冊為請求域生命周期，所以單次請求內相應對象不會銷毀，緩存不會清空，會一直保持一個。

點擊查看代碼 OptionsManager

public class OptionsManager<[DynamicallyAccessedMembers(Options.DynamicallyAccessedMembers)] TOptions> :
	IOptions<TOptions>,
	IOptionsSnapshot<TOptions>
	where TOptions : class
{
	private readonly IOptionsFactory<TOptions> _factory;
	private readonly OptionsCache<TOptions> _cache = new OptionsCache<TOptions>(); // Note: this is a private cache

	/// <summary>
	/// Initializes a new instance with the specified options configurations.
	/// </summary>
	/// <param name="factory">The factory to use to create options.</param>
	public OptionsManager(IOptionsFactory<TOptions> factory)
	{
		_factory = factory;
	}

	/// <summary>
	/// The default configured <typeparamref name="TOptions"/> instance, equivalent to Get(Options.DefaultName).
	/// </summary>
	public TOptions Value => Get(Options.DefaultName);

	/// <summary>
	/// Returns a configured <typeparamref name="TOptions"/> instance with the given <paramref name="name"/>.
	/// </summary>
	public virtual TOptions Get(string name)
	{
		name = name ?? Options.DefaultName;

		if (!_cache.TryGetValue(name, out TOptions options))
		{
			// Store the options in our instance cache. Avoid closure on fast path by storing state into scoped locals.
			IOptionsFactory<TOptions> localFactory = _factory;
			string localName = name;
			options = _cache.GetOrAdd(name, () => localFactory.Create(localName));
		}

		return options;
	}
}

IOptionsMonitor、OptionsMonitor

IOptionsMonitor 每次獲取選項類都是最新的值，它實現類是 OptionsMonitor，實現類中使用了從容器中註入的單例緩存 IOptionsMonitorCache 來保存選項類，並且通過相應的 IOptionsChangeTokenSource 註冊了選項類綁定內容的監聽，例如上面講到的 ConfigurationChangeTokenSource，在選項類配置內容改變的時候會觸發事件，而在事件中會將緩存先情況並重新獲取創建類，並且執行註冊進來的額外的監聽事件，可以看看下麵的 InvokeChanged 方法。

點擊查看代碼 OptionsMonitor

public class OptionsMonitor<[DynamicallyAccessedMembers(Options.DynamicallyAccessedMembers)] TOptions> :
	IOptionsMonitor<TOptions>,
	IDisposable
	where TOptions : class
{
	private readonly IOptionsMonitorCache<TOptions> _cache;
	private readonly IOptionsFactory<TOptions> _factory;
	private readonly List<IDisposable> _registrations = new List<IDisposable>();
	internal event Action<TOptions, string> _onChange;

	/// <summary>
	/// Constructor.
	/// </summary>
	/// <param name="factory">The factory to use to create options.</param>
	/// <param name="sources">The sources used to listen for changes to the options instance.</param>
	/// <param name="cache">The cache used to store options.</param>
	public OptionsMonitor(IOptionsFactory<TOptions> factory, IEnumerable<IOptionsChangeTokenSource<TOptions>> sources, IOptionsMonitorCache<TOptions> cache)
	{
		_factory = factory;
		_cache = cache;

		void RegisterSource(IOptionsChangeTokenSource<TOptions> source)
		{
			IDisposable registration = ChangeToken.OnChange(
					  () => source.GetChangeToken(),
					  (name) => InvokeChanged(name),
					  source.Name);

			_registrations.Add(registration);
		}

		// The default DI container uses arrays under the covers. Take advantage of this knowledge
		// by checking for an array and enumerate over that, so we don't need to allocate an enumerator.
		if (sources is IOptionsChangeTokenSource<TOptions>[] sourcesArray)
		{
			foreach (IOptionsChangeTokenSource<TOptions> source in sourcesArray)
			{
				RegisterSource(source);
			}
		}
		else
		{
			foreach (IOptionsChangeTokenSource<TOptions> source in sources)
			{
				RegisterSource(source);
			}
		}
	}

	private void InvokeChanged(string name)
	{
		name = name ?? Options.DefaultName;
		_cache.TryRemove(name);
		TOptions options = Get(name);
		if (_onChange != null)
		{
			_onChange.Invoke(options, name);
		}
	}

	/// <summary>
	/// The present value of the options.
	/// </summary>
	public TOptions CurrentValue
	{
		get => Get(Options.DefaultName);
	}

	/// <summary>
	/// Returns a configured <typeparamref name="TOptions"/> instance with the given <paramref name="name"/>.
	/// </summary>
	public virtual TOptions Get(string name)
	{
		name = name ?? Options.DefaultName;
		return _cache.GetOrAdd(name, () => _factory.Create(name));
	}

	/// <summary>
	/// Registers a listener to be called whenever <typeparamref name="TOptions"/> changes.
	/// </summary>
	/// <param name="listener">The action to be invoked when <typeparamref name="TOptions"/> has changed.</param>
	/// <returns>An <see cref="IDisposable"/> which should be disposed to stop listening for changes.</returns>
	public IDisposable OnChange(Action<TOptions, string> listener)
	{
		var disposable = new ChangeTrackerDisposable(this, listener);
		_onChange += disposable.OnChange;
		return disposable;
	}

	/// <summary>
	/// Removes all change registration subscriptions.
	/// </summary>
	public void Dispose()
	{
		// Remove all subscriptions to the change tokens
		foreach (IDisposable registration in _registrations)
		{
			registration.Dispose();
		}

		_registrations.Clear();
	}

	internal sealed class ChangeTrackerDisposable : IDisposable
	{
		private readonly Action<TOptions, string> _listener;
		private readonly OptionsMonitor<TOptions> _monitor;

		public ChangeTrackerDisposable(OptionsMonitor<TOptions> monitor, Action<TOptions, string> listener)
		{
			_listener = listener;
			_monitor = monitor;
		}

		public void OnChange(TOptions options, string name) => _listener.Invoke(options, name);

		public void Dispose() => _monitor._onChange -= OnChange;
	}
}

OnChange 方法中傳入的委托本來可以可以直接追加到事件中的，這裡將其再包裝多一層，是為了 OptionsMonitor 對象銷毀的時候能夠將相應的事件釋放，如果不包裝多一層的話，委托只在方法作用域中，對象釋放的時候是獲取不到的。

IOptionsMonitorCache、OptionsCache

OptionsCache 是 IOptionsMonitorCache 介面的的實現類，從上面可以看到 OptionsMonitor 和 OptionsSnapshot 都使用到了這個，OptionsSnapshot 通過內部創建的私有的緩存屬性實現了請求域內選項類不變，而 OptionsMonitor 則通過它減少了每次都直接讀取配置來源(如文件、資料庫、配置中心api)的性能消耗，而是通過變更事件的方式進行更新。其實我們還可以在需要的時候註入IOptionsMonitorCache 服務自行對選項類進行更新。

OptionsCache 的具體實現比較簡單，主要就是通過 ConcurrentDictionary<string, Lazy> 對象作為記憶體緩存，其中為了性能還再使用了 Lazy 方式。

IOptionsFactory、OptionsFactory

OptionsFactory 類實現 IOptionsFactory 介面，是選項類的實際創建配置之處，其實就是將之前註冊到容器中與當前相關的各種配置、驗證的行為配置類註入進來，再通過放射創建對象之後，將選項類對象傳進去，逐一對相應的行為進行調用，最後得到一個成型的選項類。這裡選項類的創建方式很簡單，這也是要求選項類要有無參構造函數的原因。

點擊查看代碼 OptionsFactory

public class OptionsFactory<[DynamicallyAccessedMembers(Options.DynamicallyAccessedMembers)] TOptions> :
	IOptionsFactory<TOptions>
	where TOptions : class
{
	private readonly IConfigureOptions<TOptions>[] _setups;
	private readonly IPostConfigureOptions<TOptions>[] _postConfigures;
	private readonly IValidateOptions<TOptions>[] _validations;

	/// <summary>
	/// Initializes a new instance with the specified options configurations.
	/// </summary>
	/// <param name="setups">The configuration actions to run.</param>
	/// <param name="postConfigures">The initialization actions to run.</param>
	public OptionsFactory(IEnumerable<IConfigureOptions<TOptions>> setups, IEnumerable<IPostConfigureOptions<TOptions>> postConfigures) : this(setups, postConfigures, validations: Array.Empty<IValidateOptions<TOptions>>())
	{ }

	/// <summary>
	/// Initializes a new instance with the specified options configurations.
	/// </summary>
	/// <param name="setups">The configuration actions to run.</param>
	/// <param name="postConfigures">The initialization actions to run.</param>
	/// <param name="validations">The validations to run.</param>
	public OptionsFactory(IEnumerable<IConfigureOptions<TOptions>> setups, IEnumerable<IPostConfigureOptions<TOptions>> postConfigures, IEnumerable<IValidateOptions<TOptions>> validations)
	{
		// The default DI container uses arrays under the covers. Take advantage of this knowledge
		// by checking for an array and enumerate over that, so we don't need to allocate an enumerator.
		// When it isn't already an array, convert it to one, but don't use System.Linq to avoid pulling Linq in to
		// small trimmed applications.

		_setups = setups as IConfigureOptions<TOptions>[] ?? new List<IConfigureOptions<TOptions>>(setups).ToArray();
		_postConfigures = postConfigures as IPostConfigureOptions<TOptions>[] ?? new List<IPostConfigureOptions<TOptions>>(postConfigures).ToArray();
		_validations = validations as IValidateOptions<TOptions>[] ?? new List<IValidateOptions<TOptions>>(validations).ToArray();
	}

	/// <summary>
	/// Returns a configured <typeparamref name="TOptions"/> instance with the given <paramref name="name"/>.
	/// </summary>
	public TOptions Create(string name)
	{
		TOptions options = CreateInstance(name);
		foreach (IConfigureOptions<TOptions> setup in _setups)
		{
			if (setup is IConfigureNamedOptions<TOptions> namedSetup)
			{
				namedSetup.Configure(name, options);
			}
			else if (name == Options.DefaultName)
			{
				setup.Configure(options);
			}
		}
		foreach (IPostConfigureOptions<TOptions> post in _postConfigures)
		{
			post.PostConfigure(name, options);
		}

		if (_validations != null)
		{
			var failures = new List<string>();
			foreach (IValidateOptions<TOptions> validate in _validations)
			{
				ValidateOptionsResult result = validate.Validate(name, options);
				if (result is not null && result.Failed)
				{
					failures.AddRange(result.Failures);
				}
			}
			if (failures.Count > 0)
			{
				throw new OptionsValidationException(name, typeof(TOptions), failures);
			}
		}

		return options;
	}

	/// <summary>
	/// Creates a new instance of options type
	/// </summary>
	protected virtual TOptions CreateInstance(string name)
	{
		return Activator.CreateInstance<TOptions>();
	}
}

以上就是 .NET Core 下的選項系統，由於選項系統的源碼不多，這裡也就將大部分類都拿出來講了一下，相當於把這個框架的流程思路都講了一遍，不知不覺寫得字數又很多了，希望有童鞋能夠耐心地看到這裡。

參考文章：
ASP.NET Core 中的選項模式 | Microsoft Learn
選項模式 - .NET | Microsoft Learn
面向 .NET 庫創建者的選項模式指南 - .NET | Microsoft Learn
理解ASP.NET Core - 選項(Options)

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