.NET Core 选项系统的主要实现在 Microsoft.Extensions.Options 和 Microsoft.Extensions.Options.ConfigurationExtensions 两个 Nuget 包。对于一个框架的源码进行解读，我们可以从我们常用的框架中的类或方法入手，这些类或方法就是我们解读的入口。

从上面对选项系统的介绍中，大家也可以看出，日常对选项系统的使用涉及到的主要有 Configure 方法，有 IOptions、IOptionsSnapshot、IOptionMonitor 等接口。

Configure

首先看选项注册，也就是 Configure 方法，注册相关的方法都是扩展方法，上面也讲到 Configure 方法有多个扩展来源，其中最常用的是 OptionsConfigurationServiceCollectionExtensions 中的 Configure 方法，该方法用于从配置信息中读取配置并绑定为选项，如下，这里将相应的方法单独摘出来了。

点击查看代码 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 接口是用来监听配置变化的服务，这个后面讲。

另外还有 OptionsServiceCollectionExtensions 中的 Configure 方法，用于直接通过委托对选项类进行配置。

点击查看代码 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，其实选项系统中的选项都是命名模式的，默认名称为 Options.Options.DefaultName，实际就是 string.Empty。当我们调用 Configure 方法对选项进行配置的时候，实际上时调用了 AddOptions 方法，并且往容器中添加了一个单例的实现了 IConfigureOptions 接口的实现。

IConfigureOptions、IConfigureNamedOptions、IPostConfigureOptions

其中 IConfigureOptions 是选项配置行为服务接口，ConfigureOptions 是它的默认实现，该类的内容很简单，它的内部主要就是保存了一个委托，用于记录使用者对选项的配置操作。

点击查看代码 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 继承了 IConfigureNamedOptions 接口，默认实现是 ConfigureNamedOptions ，作用一样，只不过多了一个方法用于应对命名选项模式。它有多个重载泛型重载，也是之前的文章ASP.NET Core - 选型系统之选型配置中讲到的“使用DI服务配置选项”的具体实现。

点击查看代码 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> 类是 IConfigureNamedOptions<TOptions> 的另一个实现，继承了ConfigureNamedOptions<TOptions> 类，重写了一些行为，最终是通过之前讲到的 ConfigurationBuilder的 Bind 方法将配置绑定到选项类而已。

<details>
<summary>点击查看代码 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 接口也是一样套路，当我们通过相应的方法传入委托对选项类进行配置的时候，会向容器中注入一个单例服务，将配置行为保存起来。

接着往下看 AddOptions 方法，AddOptions 方法有两个重载：

点击查看代码 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>

这里可以看出两者的返回值不同，而且第二个方法也调用了第一个方法，第一个方法中主要就是向容器中添加我们常用的IOptions<TOptions>、IOptionsSnapshot<TOptions>、IOptionsMonitor<TOptions> 服务接口，这里也可以看到不同服务接口对于的生命周期。除此之外还有工厂服务IOptionsFactory<>和缓存服务IOptionsMonitorCache<>，这两个就是选项体系的关键。每个选项进行配置的时候都会同时注入这些服务，所以每一个选项我们都能使用三个不同接口去解析。

# OptionsBuilder

上面第二个 AddOptions 方法返回 OptionsBuilder<TOptions> 对象。之前讲过 OptionsBuilder<TOptions> 类中也有 Configure 方法，其实不止 Configure 方法，其他的 PostConfigure 方法等也有，它其实就是最终的选项系统配置类，我们所有的选项配置其实都可以通过调用第二个 AddOptions 方法，再通过 OptionsBuilder<TOptions> 对象中的方法来完成配置。其他各个扩展方法的配置方式不过是进行了使用简化而已。

<details>
<summary>点击查看代码 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

我们除了可以对选项进行配置绑定之外，还可以对选项进行验证。验证规则是通过上面的第二个 AddOptions 方法返回的 OptionsBuilder 方法进行添加的。

验证规则配置有三种方式，最后其实都是通过 IValidateOptions 的实现类来完成。我们自己实现的自定义验证类就不用说了，最后我们会将其注入到容器中，而从上面的代码中可以看到，当我们通过委托的方式自定义验证规则的时候，它会被构建成一个 ValidateOptions 类对象，并注入到容器中作为一个服务。

ValidateOptions 是 IValidateOptions 的一个实现类，构造函数中接收委托，通过委托返回的 bool 结构判断验证是否通过。

点击查看代码 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;
	}
}

我们可以通过重载方法传入相应的验证失败提醒文本。

Options、UnnamedOptionsManager

接下来看选项使用相关的内容，其中 IOptions 中的选项类一经创建一直保持不变，默认实现类 UnnamedOptionsManager。

点击查看代码 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 接口只有一个 Value 属性，实现类中通过锁确保创建的 Value 值不会因为线程问题导致不同，且该服务被注册为单例生命周期，所以对象不销毁，后续一直会读取内存中的 Value 值。具体选项类对象的创建由工厂服务负责。

IOptionsSnapshot、OptionsManager

IOptionsSnapshot 的实现类是 OptionsManager，该类中有一个私有的 OptionsCache 属性，每次对选项类进行读取的时候，都是先尝试从缓存读取，如果没有才创建。而由于 IOptionsSnapshot 被注册为请求域生命周期，所以单次请求内相应对象不会销毁，缓存不会清空，会一直保持一个。

点击查看代码 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 每次获取选项类都是最新的值，它实现类是 OptionsMonitor，实现类中使用了从容器中注入的单例缓存 IOptionsMonitorCache 来保存选项类，并且通过相应的 IOptionsChangeTokenSource 注册了选项类绑定内容的监听，例如上面讲到的 ConfigurationChangeTokenSource，在选项类配置内容改变的时候会触发事件，而在事件中会将缓存先情况并重新获取创建类，并且执行注册进来的额外的监听事件，可以看看下面的 InvokeChanged 方法。

点击查看代码 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 方法中传入的委托本来可以可以直接追加到事件中的，这里将其再包装多一层，是为了 OptionsMonitor 对象销毁的时候能够将相应的事件释放，如果不包装多一层的话，委托只在方法作用域中，对象释放的时候是获取不到的。

IOptionsMonitorCache、OptionsCache

OptionsCache 是 IOptionsMonitorCache 接口的的实现类，从上面可以看到 OptionsMonitor 和 OptionsSnapshot 都使用到了这个，OptionsSnapshot 通过内部创建的私有的缓存属性实现了请求域内选项类不变，而 OptionsMonitor 则通过它减少了每次都直接读取配置来源(如文件、数据库、配置中心api)的性能消耗，而是通过变更事件的方式进行更新。其实我们还可以在需要的时候注入IOptionsMonitorCache 服务自行对选项类进行更新。

OptionsCache 的具体实现比较简单，主要就是通过 ConcurrentDictionary<string, Lazy> 对象作为内存缓存，其中为了性能还再使用了 Lazy 方式。

IOptionsFactory、OptionsFactory

OptionsFactory 类实现 IOptionsFactory 接口，是选项类的实际创建配置之处，其实就是将之前注册到容器中与当前相关的各种配置、验证的行为配置类注入进来，再通过放射创建对象之后，将选项类对象传进去，逐一对相应的行为进行调用，最后得到一个成型的选项类。这里选项类的创建方式很简单，这也是要求选项类要有无参构造函数的原因。

点击查看代码 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 下的选项系统，由于选项系统的源码不多，这里也就将大部分类都拿出来讲了一下，相当于把这个框架的流程思路都讲了一遍，不知不觉写得字数又很多了，希望有童鞋能够耐心地看到这里。

参考文章：
ASP.NET Core 中的选项模式 | Microsoft Learn
选项模式 - .NET | Microsoft Learn
面向 .NET 库创建者的选项模式指南 - .NET | Microsoft Learn
理解ASP.NET Core - 选项(Options)

ASP.NET Core 系列：

目录：ASP.NET Core 系列总结
上一篇：ASP.NET Core - 选项系统之选项验证

ASP.NET Core - 选项系统之源码介绍 - 啊晚

Configure

IConfigureOptions、IConfigureNamedOptions、IPostConfigureOptions

IValidateOptions

Options、UnnamedOptionsManager

IOptionsSnapshot、OptionsManager

IOptionsMonitor、OptionsMonitor

IOptionsMonitorCache、OptionsCache

IOptionsFactory、OptionsFactory

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