（13）ASP.NET Core 中的選項模式（Options）

1.前言選項（Options）模式是對配置（Configuration）的功能的延伸。在12章（ASP.NET Core中的配置二）Configuration中有介紹過該功能（綁定到實體類、綁定至對象圖、將數組綁定至類）而選項模式又有個選項類（TOptions），該選項類作用是指：把選項類中的屬性 ...

1.前言

選項（Options）模式是對配置（Configuration）的功能的延伸。在12章（ASP.NET Core中的配置二）Configuration中有介紹過該功能（綁定到實體類、綁定至對象圖、將數組綁定至類）而選項模式又有個選項類（TOptions），該選項類作用是指：把選項類中的屬性與配置來源中的鍵關聯起來。舉個例，假設json文件有個Option1鍵，選項類中也有個叫Option1的屬性名，經過選項配置，這樣就能把json中的鍵的值映射到選項類屬性值中。也可以理解在項目應用中，把一個json文件序列化到.net類。

2.常規選項配置

選項類必須為包含公共無參數構造函數的非抽象類。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類(Models/MyOptions.cs)，以下類MyOptions具有三種屬性：Option1和 Option2。設置預設值為可選，但以下示例中的類構造函數設置了Option1的預設值。Option2具有通過直接初始化屬性設置的預設值：

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類通過Configure添加到服務容器並綁定到配置：

public void ConfigureServices(IServiceCollection services)
{
    // Example #1: General configuration
    // Register the Configuration instance which MyOptions binds against.
    services.Configure<MyOptions>(Configuration);
}

也可以使用自定義ConfigurationBuilder從設置文件載入選項配置時，確認基路徑設置正確，添加到服務容器並綁定到配置：

var configBuilder = new ConfigurationBuilder()
   .SetBasePath(Directory.GetCurrentDirectory())
   .AddJsonFile("appsettings.json", optional: true);
var config = configBuilder.Build();
services.Configure<MyOptions>(config);

以下頁面模型通過IOptionsMonitor<TOptions>使用構造函數依賴關係註入來訪問設置 (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下調用IndexModel.OnGet方法返回包含選項值的字元串：

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.通過委托配置簡單選項

使用委托設置選項值。此示例應用程式使用新建MyOptionsWithDelegateConfig類 (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;
}

向服務容器添加IConfigureOptions<TOptions>服務。它通過MyOptionsWithDelegateConfig使用委托來配置綁定：

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;
    });
}

以下頁面模型通過IOptionsMonitor<TOptions>使用構造函數依賴關係註入來訪問設置 (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下調用IndexModel.OnGet方法返回包含選項值的字元串：

public HomeController(IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig)
{
    _optionsAccessorWithDelegateConfig = optionsAccessorWithDelegateConfig;
}
private readonly IOptionsMonitor<MyOptionsWithDelegateConfig> _optionsAccessorWithDelegateConfig;
public IActionResult Index()
{
    IndexModel indexModel = new IndexModel(_optionsAccessorWithDelegateConfig);
    indexModel.OnGet();
    return View();
}

每次調用Configure都會將IConfigureOptions<TOptions>服務添加到服務容器。在前面的示例中，Option1和Option2的值同時在appsettings.json中指定，但Option1和Option2的值被配置的委托替代。當啟用多個配置服務時，指定的最後一個配置源優於其他源，由其設置配置值。運行應用程式時，頁面模型的OnGet方法返回顯示選項類值的字元串。

4.子選項配置

將選項綁定到配置時，選項類型中的每個屬性都將綁定到窗體property[:sub-property:]的配置鍵。例如，MyOptions.Option1屬性將綁定到從appsettings.json中的option1屬性讀取的鍵Option1。在以下代碼中，已向服務容器添加IConfigureOptions<TOptions>服務。它將MySubOptions綁定到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類(Models/MySubOptions.cs)將屬性SubOption1和SubOption2定義為保留選項值：

public class MySubOptions
{
    public MySubOptions()
    {
        // Set default values.
        SubOption1 = "value1_from_ctor";
        SubOption2 = 5;
    }
    public string SubOption1 { get; set; }
    public int SubOption2 { get; set; }
}

以下頁面模型通過IOptionsMonitor<TOptions>使用構造函數依賴關係註入來訪問設置(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下調用IndexModel.OnGet方法返回包含選項值的字元串：

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.通過IOptionsSnapshot重新載入配置數據

IOptionsSnapshot針對請求生命周期訪問和緩存選項時，每個請求只能計算一次選項。以下示例演示如何在更改appsettings.json(Pages/Index.cshtml.cs)後創建新的 IOptionsSnapshot<TOptions>。在更改appsettings.json文件和重新載入配置之前，針對伺服器的多個請求返回appsettings.json文件提供的配置鍵值。

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}";
    }
}

下麵顯示從appsettings.json文件載入的初始option1和option2值：

snapshot option1 = value1_from_json, snapshot option2 = -1

將appsettings.json文件中的值更改為value1_from_json UPDATED和200。保存appsettings.json 文件。刷新瀏覽器，查看更新的選項值：

snapshot option1 = value1_from_json UPDATED, snapshot option2 = 200

6.包含IConfigureNamedOptions的命名選項支持

命名選項支持允許應用程式在命名選項配置之間進行區分。命名選項通過OptionsServiceCollectionExtensions.Configure進行聲明，其調用擴展方法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";
    });
}

通過OnGet(Pages/Index.cshtml.cs)訪問命名選項：

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下調用IndexModel.OnGet方法返回包含選項值的字元串：

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方法配置所有選項

使用ConfigureAll方法可以配置所有選項實例。以下代碼將針對包含公共值的所有配置實例配置Option1。將以下代碼手動添加到Startup.ConfigureServices方法：

services.ConfigureAll<MyOptions>(myOptions =>
{
    myOptions.Option1 = "ConfigureAll replacement value";
});

在Home/Index控制器Action下調用IndexModel.OnGet方法返回包含選項值的字元串：

參考文獻：
ASP.NET Core 中的選項模式