如何获取 appsettings.json 里的值

一、比如appsettings.json里面有这样一段代码

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*",
  "ConnectionStrings": {
    "ErpConnection": "server=192.111.1.1;database=Erp;uid=sa;pwd=123;MultipleActiveResultSets=true;"
  }
}

二、在Startup 类中，我们需要读取 数据库连接字符串 ErpConnection，可以这样

public class Startup
{
        //注入配置类
        public IConfiguration Configuration { get; }
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }
        public void ConfigureServices(IServiceCollection services)
        {
           services.AddDbContext<Dbs.Models.ErpContext>(options =>
            {
                //2种方法都可以连接数据库
                //方法1：直接通过 GetConnectionString 方法获取节点
               //options.UseSqlServer(Configuration.GetConnectionString("ErpConnection")); 

               //方法2：通过GetSection 一级一级获取节点
               //options.UseSqlServer(Configuration.GetSection("ConnectionStrings").GetSection("ErpConnection").Value); 
               options.UseSqlServer(Configuration.GetSection("ConnectionStrings:ErpConnection").Value); 
           }, ServiceLifetime.Transient); 
        } 
}

三、使用GetSection()获取复杂的值

比如我们在appsettings.json中有这样一段配置

{
"AppConnectionStrings": {
    "PathPrefix": "",
    "Something": "Something context",
    "Clients": { "line": "longclients" }
  }
}

如果我们想要获取Clients的值，我们可能会这样操作

var clients = Configuration.GetSection("AppConnectionStrings:Clients").Value;

结果返回的 clients 是null。
其实我们正确的做法是这样的：
1、我们要根据json的数据格式创建相应的类

public class AppConnectionStrings
{
    public string PathPrefix { get; set; }
    public string Something { get; set; }
    public Clients Clients { get; set; }
}

public class Clients
{
    public string line { get; set; }
}

2、将Options服务添加到服务集合并从配置中加载设置

public void ConfigureServices(IServiceCollection services)
{
    // This is only required for .NET Core 2.0
    services.AddOptions();

    services.Configure<AppConnectionStrings>(Configuration.GetSection("AppConnectionStrings"));

    services.AddMvc();
}

注：也可以通过指定要加载的配置部分来加载方法中的子选项

services.Configure<Clients>(Configuration.GetSection("AppConnectionStrings:Clients");

3、控制器中我们可以通过构造函数注入IOptions<Clients>来读取

[Route("api/[controller]")]
    [ApiController]
    public class TestController : ControllerBase
    {
        private readonly AppConnectionStrings _settings;

        public TestController(IOptions<AppConnectionStrings> settings)
        {
            _settings = settings.Value;
        }

        [Route("GetAppConnectionStrings")]
        [HttpGet]
        public ActionResult<JObject> GetAppConnectionStrings()
        {
            return JObject.FromObject( _settings);
        }
    }

我们请求GetAppConnectionStrings接口的时候，就得到相应配置数据

注：使用前一定要先在Startup 类中注入服务services.Configure<AppConnectionStrings>(Configuration.GetSection("AppConnectionStrings"));
没有这一步，读取不了数据：