（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": -,
  "subsection": {
    "suboption1": "subvalue1_from_json",
    "suboption2":
  },
  "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; } = ;
}

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; } = ;
}

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 = ;
    });
}

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 = ;
    }
    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 = -

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 = 

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