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); 
        } 
}