1､使用ConfigurationBuilder读取配置项的值

1）Settings类

public class Settings
    {
        private static IHttpContextAccessor _HttpContextAccessor;
        public Settings(IHttpContextAccessor httpContextAccessor)
        {
            _HttpContextAccessor = httpContextAccessor;
        }
        public static void Configure(IHttpContextAccessor httpContextAccessor)
        {
            _HttpContextAccessor = httpContextAccessor;
        }
        public static IConfigurationBuilder Getbuilder()
        {
            var builder = new ConfigurationBuilder()
              .SetBasePath(Directory.GetCurrentDirectory())
              .AddJsonFile("appsettings.json");
            return builder;
        }
        public static string GetAppSetting(string key)
        {
            //return Convert.ToString(ConfigurationManager.AppSettings[key]);
            var builder = Getbuilder();
            var GetAppStringData = builder.Build().GetValue<string>("AppSettings:" + key);
            return GetAppStringData;
        }
        public static string GetConnectionString(string key="DefaultName")
        {
            var builder = Getbuilder();
            var ConnectionString = builder.Build().GetValue<string>("ConnectionStrings:"+key);
            return ConnectionString;
        }
    }

2）在Startup中Configure()方法配置

public class Startup
{      
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {           
        var httpContextAccessor = app.ApplicationServices.GetRequiredService<IHttpContextAccessor>();
        Settings.Configure(httpContextAccessor);
    }
}

2､使用新建配置Model的获取配置

需要使用Nuget引用如下，

 "Microsoft.Extensions.Options.ConfigurationExtensions": "1.0.0",
 "Microsoft.Extensions.Configuration.Json": "1.0.0"

搜索 "Microsoft.Extensions.Options.ConfigurationExtensions" 和 “Microsoft.Extensions.Configuration.Json”，在列表中分别找到它，点击 "安装" 即可。

相关文档：VS(Visual Studio)中Nuget的使用

1）appsettings.json配置文件

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*",
  "HangFire": {
    "Connection": "Server=127.0.0.1;uid=root;pwd=website;database=Hangfire_DB;AllowLoadLocalInfile=true;Allow User Variables=True;",
    "pathMatch": "/hangfire",
    "Login": "login",
    "PasswordClear": "pwd"
  }
}

2）AppSettingsModel类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace WebApplication1
{
    public class AppSettingsModel
    {
        public string Connection { get; set; }
        public string pathMatch { get; set; }
        public string Login { get; set; }
        public string PasswordClear { get; set; }
    }
}

3）ConfSetup配置类

public static class ConfSetup
{
    private static IConfigurationRoot _configuration;
    public static IConfigurationRoot Configuration
    {
        get
        {
            if (_configuration == null)
            {

                var builder = new ConfigurationBuilder().AddJsonFile("appsettings.json");
                _configuration = builder.Build();
               
            }
            return _configuration;
        }
        set
        {
            _configuration = value;
        }
    }
}

4）获取配置项的值

var pathMatch = ConfSetup.Configuration["HangFire:pathMatch"]
var connection = ConfSetup.Configuration["HangFire:Connection"]

5）获取配置AppSettingsModel的对象

public static void Main(string[] args)
{
    var hostBuilder = new HostBuilder()
   .UseConsoleLifetime()
   .ConfigureServices((hostContext, services) =>
   {
       services.Configure<AppSettingsModel>(ConfSetup.Configuration.GetSection("HangFire"));
   });
   
   var host = hostBuilder.Build();
     
   var opt = host.Services.GetService<IOptions<AppSettingsModel>>();
    AppSettingsModel model = opt.Value;
}

相关文档：.net core appsettings.json 获取数据库连接字符串