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1.前言
选项（Options）模式是对配置（Configuration）的功能的延伸。在12章（ASP.NET Core中的配置二）Configuration中有介绍过该功能（绑定到实体类、绑定至对象图、将数组绑定至类）而选项模式又有个选项类（TOptions），该选项类作用是指：把选项类中的属性与配置来源中的键关联起来。举个例，假设json文件有个Option1键，选项类中也有个叫Option1的属性名，经过选项配置，这样就能把json中的键的值映射到选项类属性值中。也可以理解在项目应用中，把一个json文件序列化到.net类。
2.常规选项配置
选项类必须为包含公共无参数构造函数的非抽象类。在appsettings.json文件中添加option1、option2、subsection的配置：
{ "option1": "value1_from_json", "option2": -1, "subsection": { "suboption1": "subvalue1_from_json", "suboption2": 200 }, "Logging": { "LogLevel": { "Default": "Warning" } }, "AllowedHosts": "*"}新建MyOptions类(Models/MyOptions.cs)，以下类MyOptions具有三种属性：Option1和 Option2。设置默认值为可选，但以下示例中的类构造函数设置了Option1的默认值。Option2具有通过直接初始化属性设置的默认值：
public class MyOptions{ public MyOptions() { // Set default value. Option1 = "value1_from_ctor"; } public string Option1 { get; set; } public int Option2 { get; set; } = 5;}而MyOptions类通过Configure添加到服务容器并绑定到配置：
public void ConfigureServices(IServiceCollection services){ // Example #1: General configuration // Register the Configuration instance which MyOptions binds against. services.Configure<MyOptions>(Configuration);}也可以使用自定义ConfigurationBuilder从设置文件加载选项配置时，确认基路径设置正确，添加到服务容器并绑定到配置：
var configBuilder = new ConfigurationBuilder() .SetBasePath(Directory.GetCurrentDirectory()) .AddJsonFile("appsettings.json", optional: true);var config = configBuilder.Build();services.Configure<MyOptions>(config);以下页面模型通过IOptionsMonitor<TOptions>使用构造函数依赖关系注入来访问设置 (Pages/Index.cshtml.cs)：
public class IndexModel{ public IndexModel(IOptionsMonitor<MyOptions> optionsAccessor) { _options = optionsAccessor.CurrentValue; } private readonly MyOptions _options; public void OnGet() { // Example #1: Simple options var option1 = _options.Option1; var option2 = _options.Option2; var simpleOptions = $"option1 = {option1}, option2 = {option2}"; }}在Home/Index控制器Action下调用IndexModel.OnGet方法返回包含选项值的字符串：
public HomeController(IOptionsMonitor<MyOptions> optionsAccessor){ _optionsAccessor = optionsAccessor;}private readonly IOptionsMonitor<MyOptions> _optionsAccessor;public IActionResult Index(){ IndexModel indexModel = new IndexModel(_optionsAccessor); indexModel.OnGet(); return View();}
3.通过委托配置简单选项
使用委托设置选项值。此示例应用程序使用新建MyOptionsWithDelegateConfig类 (Models/MyOptionsWithDelegateConfig.cs)：
public class MyOptionsWithDelegateConfig{ public MyOptionsWithDelegateConfig() { // Set default value. Option1 = "value1_from_ctor"; } public string Option1 { get; set; } public int Option2 { get; set; } = 5;}向服务容器添加IConfigureOptions<TOptions>服务。它通过MyOptionsWithDelegateConfig使用委托来配置绑定：
public void ConfigureServices(IServiceCollection services){ // Example #2: Options bound and configured by a delegate services.Configure<MyOptionsWithDelegateConfig>(myOptions => { myOptions.Option1 = "value1_configured_by_delegate"; myOptions.Option2 = 500; });}以下页面模型通过IOptionsMonitor<TOptions>使用构造函数依赖关系注入来访问设置 (Pages/Index.cshtml.cs)：
public class IndexModel{ private readonly MyOptionsWithDelegateConfig _optionsWithDelegateConfig; public IndexModel(IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig) { _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue; } public void OnGet() { // Example #2: Options configured by delegate var delegate_config_option1 = _optionsWithDelegateConfig.Option1; var delegate_config_option2 = _optionsWithDelegateConfig.Option2; var simpleOptionsWithDelegateConfig = $"delegate_option1 = {delegate_config_option1}, " + $"delegate_option2 = {delegate_config_option2}"; }}在Home/Index控制器Action下调用IndexModel.OnGet方法返回包含选项值的字符串：
public HomeController(IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig){ _optionsAccessorWithDelegateConfig = optionsAccessorWithDelegateConfig;}private readonly IOptionsMonitor<MyOptionsWithDelegateConfig> _optionsAccessorWithDelegateConfig;public IActionResult Index(){ IndexModel indexModel = new IndexModel(_optionsAccessorWithDelegateConfig); indexModel.OnGet(); return View();}
每次调用Configure都会将IConfigureOptions<TOptions>服务添加到服务容器。在前面的示例中，Option1和Option2的值同时在appsettings.json中指定，但Option1和Option2的值被配置的委托替代。当启用多个配置服务时，指定的最后一个配置源优于其他源，由其设置配置值。运行应用程序时，页面模型的OnGet方法返回显示选项类值的字符串。
4.子选项配置
将选项绑定到配置时，选项类型中的每个属性都将绑定到窗体property[:sub-property:]的配置键。例如，MyOptions.Option1属性将绑定到从appsettings.json中的option1属性读取的键Option1。在以下代码中，已向服务容器添加IConfigureOptions<TOptions>服务。它将MySubOptions绑定到appsettings.json文件的subsection部分：
public void ConfigureServices(IServiceCollection services){ // Example #3: Suboptions // Bind options using a sub-section of the appsettings.json file. services.Configure<MySubOptions>(Configuration.GetSection("subsection"));}新建MySubOptions类(Models/MySubOptions.cs)将属性SubOption1和SubOption2定义为保留选项值：
public class MySubOptions{ public MySubOptions() { // Set default values. SubOption1 = "value1_from_ctor"; SubOption2 = 5; } public string SubOption1 { get; set; } public int SubOption2 { get; set; }}以下页面模型通过IOptionsMonitor<TOptions>使用构造函数依赖关系注入来访问设置(Pages/Index.cshtml.cs)：
public class IndexModel{ private readonly MySubOptions _subOptions; public IndexModel(IOptionsMonitor<MySubOptions> subOptionsAccessor) { _subOptions = subOptionsAccessor.CurrentValue; } public void OnGet() { // Example #3: Suboptions var subOption1 = _subOptions.SubOption1; var subOption2 = _subOptions.SubOption2; var subOptions = $"subOption1 = {subOption1}, subOption2 = {subOption2}"; }}在Home/Index控制器Action下调用IndexModel.OnGet方法返回包含选项值的字符串：
public HomeController(IOptionsMonitor<MySubOptions> subOptionsAccessor){ _subOptionsAccessor = subOptionsAccessor;}private readonly IOptionsMonitor<MySubOptions> _subOptionsAccessor;public IActionResult Index(){ IndexModel indexModel = new IndexModel(_subOptionsAccessor); indexModel.OnGet(); return View();}
5.通过IOptionsSnapshot重新加载配置数据
IOptionsSnapshot针对请求生命周期访问和缓存选项时，每个请求只能计算一次选项。以下示例演示如何在更改appsettings.json(Pages/Index.cshtml.cs)后创建新的 IOptionsSnapshot<TOptions>。在更改appsettings.json文件和重新加载配置之前，针对服务器的多个请求返回appsettings.json文件提供的配置键值。
public class IndexModel{ private readonly MyOptions _snapshotOptions; public IndexModel(IOptionsSnapshot<MyOptions> snapshotOptionsAccessor) { _snapshotOptions = snapshotOptionsAccessor.Value; } public void OnGet() { // Example #5: Snapshot options var snapshotOption1 = _snapshotOptions.Option1; var snapshotOption2 = _snapshotOptions.Option2; var snapshotOptions = $"snapshot option1 = {snapshotOption1}, " + $"snapshot option2 = {snapshotOption2}"; }}下面显示从appsettings.json文件加载的初始option1和option2值：
snapshot option1 = value1_from_json, snapshot option2 = -1将appsettings.json文件中的值更改为value1_from_json UPDATED和200。保存appsettings.json 文件。刷新浏览器，查看更新的选项值：
snapshot option1 = value1_from_json UPDATED, snapshot option2 = 2006.包含IConfigureNamedOptions的命名选项支持
命名选项支持允许应用程序在命名选项配置之间进行区分。命名选项通过OptionsServiceCollectionExtensions.Configure进行声明，其调用扩展方法ConfigureNamedOptions<TOptions>.Configure：
public void ConfigureServices(IServiceCollection services){ // Example #6: Named options (named_options_1) // Register the ConfigurationBuilder instance which MyOptions binds against. // Specify that the options loaded from configuration are named // "named_options_1". services.Configure<MyOptions>("named_options_1", Configuration); // Example #6: Named options (named_options_2) // Specify that the options loaded from the MyOptions class are named // "named_options_2". // Use a delegate to configure option values. services.Configure<MyOptions>("named_options_2", myOptions => { myOptions.Option1 = "named_options_2_value1_from_action"; });}通过OnGet(Pages/Index.cshtml.cs)访问命名选项：
public class IndexModel{ private readonly MyOptions _named_options_1; private readonly MyOptions _named_options_2; public IndexModel(IOptionsSnapshot<MyOptions> namedOptionsAccessor) { _named_options_1 = namedOptionsAccessor.Get("named_options_1"); _named_options_2 = namedOptionsAccessor.Get("named_options_2"); } public void OnGet() { // Example #6: Named options var named_options_1 = $"named_options_1: option1 = {_named_options_1.Option1}, " + $"option2 = {_named_options_1.Option2}"; var named_options_2 = $"named_options_2: option1 = {_named_options_2.Option1}, " + $"option2 = {_named_options_2.Option2}"; var namedOptions = $"{named_options_1} {named_options_2}"; }}在Home/Index控制器Action下调用IndexModel.OnGet方法返回包含选项值的字符串：
public HomeController(IOptionsSnapshot<MyOptions> namedOptionsAccessor){ _namedOptionsAccessor = namedOptionsAccessor;}private readonly IOptionsSnapshot<MyOptions> _namedOptionsAccessor;public IActionResult Index(){ IndexModel indexModel = new IndexModel(_namedOptionsAccessor); indexModel.OnGet(); return View();}
使用ConfigureAll方法配置所有选项
使用ConfigureAll方法可以配置所有选项实例。以下代码将针对包含公共值的所有配置实例配置Option1。将以下代码手动添加到Startup.ConfigureServices方法：
services.ConfigureAll<MyOptions>(myOptions =>{ myOptions.Option1 = "ConfigureAll replacement value";});在Home/Index控制器Action下调用IndexModel.OnGet方法返回包含选项值的字符串：
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