[译]ASP.NET Core 2.0 全局配置项

问题

如何在 ASP.NET Core 2.0 应用程序中读取全局配置项?

答案

首先新建一个空项目,并添加两个配置文件:

1. appsettings.json

{
  "Section1": {
    "SettingA": "ValueA",
    "SettingB": "ValueB"
  },
  "Section2": {
    "SettingC": "ValueC"
  }
}

2. appsettings.Development.json

{
  "Section1": {
    "SettingA": "Dev_ValueA"
  },
  "Section2": {
    "SettingC": "Dev_ValueC"
  }
}

Visual Studio会自动识别两者的关系,并在解决方案层次结构中展示如下:

然后创建相应的POCO类,分别对应于几个配置节点:

public class AppSettings
{
    public AppSettingsSection1 Section1 { get; set; }
    public AppSettingsSection2 Section2 { get; set; }
}

public class AppSettingsSection1
{
    public string SettingA { get; set; }
    public string SettingB { get; set; }
}

public class AppSettingsSection2
{
    public string SettingC { get; set; }
}

在Startup.cs文件中,创建接收 IConfiguration 的构造函数:

public static IConfiguration Configuration { get; private set;}

public Startup(IConfiguration config)
{
    Configuration = config;
}

然后在 ConfigureServices() 方法中添加Options服务,并设置依赖项:

public void ConfigureServices(IServiceCollection services)
{
    services.AddOptions();
    services.Configure<AppSettings>(Configuration);
}

最后,将配置项作为IOptions接口注入中间件的构造函数,其中泛型类型T就是我们刚才定义的POCO类:

public class HelloWorldMiddleware
{
    private readonly RequestDelegate _next;
    private readonly AppSettings _settings;

    public HelloWorldMiddleware(RequestDelegate next, IOptions<AppSettings> options)
    {
        _next = next;
        _settings = options.Value;
    }

    public async Task Invoke(HttpContext context)
    {
        var jsonSettings = JsonConvert.SerializeObject(_settings, Formatting.Indented);
        await context.Response.WriteAsync(jsonSettings);
    }
}


public static class UseHelloWorldInClassExtensions
{
    public static IApplicationBuilder UseHelloWorld(this IApplicationBuilder app)
    {
        return app.UseMiddleware<HelloWorldMiddleware>();
    }
}

在Startup.cs的 Configure() 方法中,将此中间件注入到请求管道中:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseHelloWorld();
}

运行,此时页面显示:

讨论

 ASP.NET Core 拥有一个简单的机制来从各种数据源(比如JSON文件,环境变量,甚至是自定义数据源)中读取应用程序设置。然后通过依赖注入,方便的使用这些配置项。

尽管这一切看起来很魔幻(我们的设置究竟是如何加载的!),ASP.NET Core 2.0隐藏了从数据源中读取配置项的细节,这些内容本应该存在于Program.cs文件中WebHost的CreateDefaultBuilder()方法中。IConfiguration随后被添加到服务容器中,并在应用程序的其他部分保持可用,我们使用Startup中的此接口来添加配置项。为了观察这个过程,请将Program.cs文件中的BuildWebHost()方法替换为如下内容,得到的结果是一样的:

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .ConfigureAppConfiguration((context, builder) =>
        {
            var env = context.HostingEnvironment;

            builder.AddJsonFile("appsettings.json",
                         optional: true, reloadOnChange: true)
                   .AddJsonFile($"appsettings.{env.EnvironmentName}.json",
                         optional: true, reloadOnChange: true);

            if (env.IsDevelopment())
            {
                var appAssembly = Assembly.Load(
                    new AssemblyName(env.ApplicationName));
                if (appAssembly != null)
                {
                    builder.AddUserSecrets(appAssembly, optional: true);
                }
            }

            builder.AddEnvironmentVariables();

            if (args != null)
            {
                builder.AddCommandLine(args);
            }
        })
        .Build();

在上面的解决方案中,我们提供了两个JSON文件数据源。需要记着的一点是,这些文件按照顺序被依次读取,后面的数据源会覆盖前面的数据源。你也可以在上面的运行结果中注意到,SettingB配置项来自于第一个配置文件,而其他两个配置项都来自于第二个配置文件。

注意:Startup.cs中的IConfiguration实例拥有public static修饰符,因此可以在整个应用程序期间使用此实例:

var valueA = Config["Section1:SettingA"];

然而,更好的办法是将配置项读入一个类型化的POCO类,并将其作为依赖项注入中间件或者控制器。上面的示例正好展示了这个模式。

你也可以为不同的配置节定义不同的POCO类,并使用IConfiguration的GetSection()方法来读取。

 

====start by sanshi=========================

下面我们简单扩展之前的示例,来读取不同的配置节:

public void ConfigureServices(IServiceCollection services)
{
    services.AddOptions();
    services.Configure<AppSettings>(Configuration);
    services.Configure<AppSettingsSection1>(Configuration.GetSection("Section1"));
}

更新中间件代码,此时向中间件的构造函数注入两个依赖项:

public class HelloWorldMiddleware
{
    private readonly RequestDelegate _next;
    private readonly AppSettings _settings;
    private readonly AppSettingsSection1 _settingsSection1;

    public HelloWorldMiddleware(RequestDelegate next, IOptions<AppSettings> options, IOptions<AppSettingsSection1> optionsSection1)
    {
        _next = next;
        _settings = options.Value;
        _settingsSection1 = optionsSection1.Value;
    }

    public async Task Invoke(HttpContext context)
    {
        var jsonSettings = JsonConvert.SerializeObject(_settings, Formatting.Indented);
        var jsonSettingsSection1 = JsonConvert.SerializeObject(_settingsSection1, Formatting.Indented);
        await context.Response.WriteAsync("AppSettings:\n" + jsonSettings + "\n\nAppSettings - Section1:\n" + jsonSettingsSection1);
    }
}

运行,此时页面显示:

====end by sanshi=========================

 

当然,我们也可以手工设置配置项的值,通过使用IServiceCollection.Configure的重载方法并接收强类型的lambda表达式:

====start by sanshi=========================

 修改ConfigurationServices()方法,手工设置配置项:

public void ConfigureServices(IServiceCollection services)
{
    services.AddOptions();
    
    services.Configure<AppSettings>(options =>
    {
        options.Section1 = new AppSettingsSection1();
        options.Section1.SettingA = "SettingA Value";
        options.Section1.SettingB = "SettingB Value";
    });
}

运行,此时页面效果:

====end by sanshi=========================

 

源代码下载

 

原文:https://tahirnaushad.com/2017/08/15/asp-net-core-configuration/

posted @ 2017-10-25 09:29 三生石上(FineUI控件) 阅读(...) 评论(...) 编辑 收藏