选项用来提供对相关设置的强类型访问，读取配置首选使用选项模式。选项无法脱离容器使用，依赖容器，实现了选项不同的访问方式。选项模式使用了泛型包装器，因此具备了如下优点：
- 不需要显示注册选项具体类型，只需要将泛型包装器注入到容器中；
- 对于选项实例的评估推迟到获取IOptions.Value时进行，而不是在注入时进行，这样就可以获取不同生命周期的选项；
- 可以对选项进行泛型约束；
选项注入
选项模式向容器中注入了三种类型的选项泛型包装器：IOptions<>，IOptionsSnapshot<>，IOptionsMonitor<>。其中IOptionsSnapshot<>被注册为Scoped。注入了IOptionsFactory<>泛型选项工厂，用来创建选项实例。注入了IOptionsMonitorCache<>，由IOptionsMonitor<>用于缓存泛型实例。
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;
}
选项不同生命周期
不同类型的选项接口功能不同：
- IOptions<>：实现类为UnnamedOptionsManager，其中TOptions字段保存选项实例（不支持命名选项）。在首次获取Value时，会调用工厂创建选项实例。因为被注册为单例，因此无法识别配置修改。
- IOptionsSnapshot<>：实现类为OptionsManager，其中OptionsCache字段（私有，非容器获取）保存选项实例（支持命名选项）。被注册为范围，在范围内首次获取Value时，会调用工厂创建选项实例，并将其保存到私有的OptionsCache中，在范围内选项值不变，不同范围内选项值根据获取时配置文件的不同而不同。
- IOptionsMonitor<>：实现类为OptionsMonitor，其中IOptionsMonitorCache字段，该字段的值是从容器中解析的，用来缓存选项实例。OptionsMonitor还注入了IOptionsChangeTokenSource列表，可以监听配置源的修改。当监听到修改时，调用工厂重新创建选项以刷新选项值。
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);
}
}
}
}
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>();
public OptionsManager(IOptionsFactory<TOptions> factory)
{
_factory = factory;
}
public TOptions Value => Get(Options.DefaultName);
public virtual TOptions Get(string name)
{
name = name ?? Options.DefaultName;
if (!_cache.TryGetValue(name, out TOptions options))
{
IOptionsFactory<TOptions> localFactory = _factory;
string localName = name;
options = _cache.GetOrAdd(name, () => localFactory.Create(localName));
}
return options;
}
}
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;
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);
}
// 此处简写
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);
}
public TOptions CurrentValue
{
get => Get(Options.DefaultName);
}
public virtual TOptions Get(string name)
{
name = name ?? Options.DefaultName;
return _cache.GetOrAdd(name, () => _factory.Create(name));
}
public IDisposable OnChange(Action<TOptions, string> listener)
{
var disposable = new ChangeTrackerDisposable(this, listener);
_onChange += disposable.OnChange;
return disposable;
}
public void Dispose()
{
foreach (IDisposable registration in _registrations)
{
registration.Dispose();
}
_registrations.Clear();
}
}
选项配置
选项模式提供了IConfigureOptions<>、IConfigureNamedOptions<>和IPostConfigureOptions<>接口，用来对选项进行配置。IConfigureNamedOptions<>继承了IConfigureOptions<>接口，增加了命名选项配置功能。这三个接口中都有一个对选项配置的方法，将接口注入到容器中，当调用工厂创建选项时，会调用接口中的配置方法对选项进行配置。首先会调用IConfigureOptions<>、IConfigureNamedOptions<>接口中的配置方法，然后调用IPostConfigureOptions<>接口中的配置方法。
// OptionsFactory<>
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);
}
// 选项验证...
return options;
}
protected virtual TOptions CreateInstance(string name)
{
return Activator.CreateInstance<TOptions>();
}
选项验证
选项模式提供了IValidateOptions<>接口，包含一个Validate方法对选项进行验证。将接口注入容器中，当调用工厂创建选项时，会调用接口中的Validate方法对选项进行验证。
// OptionsFactory<>
public TOptions Create(string name)
{
TOptions options = CreateInstance(name);
// 选项配置...
if (_validations.Length > 0)
{
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;
}
如果需要为选项添加验证，实现IValidateOptions<>接口并注入到容器即可：
builder.Services.TryAddEnumerable(ServiceDescriptor.Singleton
<IValidateOptions<SettingsOptions>, ValidateSettingsOptions>());
也可以添加DataAnnotations验证，调用ValidateDataAnnotations扩展方法即可，该方法定义在 Microsoft.Extensions.Options.DataAnnotations中。需要先调用AddOptions<>扩展方法创建OptionsBuilder<>：
builder.Services.AddOptions<SettingsOptions>()
.Bind(Configuration.GetSection(SettingsOptions.ConfigurationSectionName))
.ValidateDataAnnotations();
绑定配置
通过如下扩展方法将选项绑定到配置：
builder.Services.Configure<SettingsOptions>(builder.Configuration.GetSection("Settings"));
绑定到配置是通过IConfiguration.Bind()扩展方法实现的，同时，也添加了对配置修改的监听：
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));
}