（13）ASP.NET Core 中的选项模式（Options）

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 = 200

6.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();
}

5.1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：

参考文献：
ASP.NET Core 中的选项模式