在没有IOptions< T>的情况下从appsettings.json中读取和使用设置吗？

问题描述

.NET Core 从配置文件中懒惰地读取设置，将其反序列化为POCO，然后使用一行代码将POCO注册到内置DI容器中：

.NET Core allows to lazily read settings from configuration file, de-serialize it to a POCO and register that POCO in built-in DI container with one line of code:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<MyOptions>(Configuration.GetSection("MySection"));
}

然后，任何消费者都可以解析 IOptions< MyOptions> 访问该POCO：

Then any consumer can resolve IOptions<MyOptions> to access that POCO:

public HomeController(IOptions<MyOptions> optionsAccessor)
{
    MyOptions options = optionsAccessor.Value;
}

这种方法有很多缺点：

• Microsoft.Extensions.Options 包中不必要的依赖项：

• Unnecessary dependency from Microsoft.Extensions.Options package:

模拟，测试和显式实例创建变得更加冗长。

Mocking, testing and explicit instance creation become a bit more verbose.

什么是没有 IOptions< T> 的最简单解决方案？

What is the easiest solution to resolve options without IOptions<T>?

推荐答案

反序列化 configuration.Get< TOptions> 或 configuration.Bind 的选项，并在DI容器中将POCO明确注册为单例：

Deserialize options with configuration.Get<TOptions> or configuration.Bind call and register a POCO in DI container explicitly as singleton:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingletonFromFile<MyOptions>(Configuration.GetSection("MySection"));
}

//...

public static IServiceCollection AddSingletonFromFile<TOptions>(
    this IServiceCollection services,
    IConfiguration configuration)
    where TOptions : class, new()
{
    //POCO is created with actual values 
    TOptions options = configuration.Get<TOptions>();

    services.AddSingleton(options);

    return services;
}

UPD：感谢@NPNelson提供的。Get< TOptions>（）提示。

UPD: thanks to @NPNelson for .Get<TOptions>() hint.

然后 IOptions< T> 解析不再

public HomeController(MyOptions options)
{
    _options = options;
}

仅供参考：也可以从外部服务（数据库等）读取方式：

FYI: reading from an external service (database etc.) is also possible this way:

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransientFromService<OptionsReader, MyOptions>(reader => reader.GetOptions());
}

//...

public static void AddTransientFromService<TReader, TOptions>(
    this IServiceCollection services,
    Func<TReader, TOptions> getOptions)
    where TReader : class
    where TOptions : class

{
    services.AddTransient(provider => getOptions(provider.GetService<TReader>()));
}

备注：

• Singleton并不懒惰（总是在启动时实例化）；


• 如果是单例注册，则在运行时会丢失任何更新选项的功能（.NET Core通过 reloadOnChange 选项设置本地支持运行时文件重新加载： .AddJsonFile（ appsettings.json，false，reloadOnChange：true））;

• Singleton is not lazy (it's always instantiated at startup);
• In case of singleton registration, any ability to update options at runtime is lost (.NET Core natively supports runtime file reloading with reloadOnChange option setup: .AddJsonFile("appsettings.json", false, reloadOnChange: true));

如果您确实需要重新加载文件，但仍然不想使用 IOptions ，请考虑临时解析。当然，按请求解决可能会导致性能显着下降。

If you really need the file reload and you still don't want to use IOptions, consider a transient resolving. Of course, per-request resolving can lead to the significant perfomance decrease.

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