1、添加引用Microsoft.Extensions.Configuration.Json

添加引用 Microsoft.Extensions.Configuration.Json，引用方法可以参考：

1）使用Nuget界面管理器

搜索 "Microsoft.Extensions.Configuration.Json" 在列表中分别找到它，点击"安装"

相关文档：VS(Visual Studio)中Nuget的使用

2）使用Package Manager命令安装

PM > Install-Package Microsoft.Extensions.Configuration.Json

3）使用.NET CLI命令安装

> dotnet add package Microsoft.Extensions.Configuration.Json

2、使用ConfigurationBuilder读取appsettings.json

1）appsettings.json内容

{
   "ConnectionString": "Server=localhost;Database=tempDB;Uid=<dbUserName>;Pwd=<dbPassword>",
   "Smtp": {
     "Host": "smtp.gmail.com",
     "From": "Levi"
   },
   "settings": {
   "From": "url@gmail.com",
   "SmtpServer": "smtp.gmail.com",
   "Port": 465,
   "Username": "url@gmail.com",
   "Password": "123456"
   }
 }

2）实现代码

var configuration =  new ConfigurationBuilder()
     .AddJsonFile($"appsettings.json");
            
var config = configuration.Build();
var connectionString = config.GetConnectionString("ConnectionString");

3、获取IConfiguration的实例来读取appsettings.json

1）appsettings.json内容

{
  "ConnectionStrings": {
    "mysql": "Server=localhost;Database=MyDatabase;Trusted_Connection=True", 
    "sqlserver"："Server=localhost;Database=MyDatabase;Trusted_Connection=True"
   },
   "settings": {
   "From": "url@gmail.com",
   "SmtpServer": "smtp.gmail.com",
   "Port": 465,
   "Username": "url@gmail.com",
   "Password": "123456"
   }
}

2）实现代码

using IHost host = Host.CreateDefaultBuilder(args).Build();

IConfiguration config = host.Services.GetRequiredService<IConfiguration>();
//一种方法
string con= config.GetValue<string>("ConnectionStrings:mysql");
//另一种方法
string connectionString = config["ConnectionStrings:sqlserver"];

Console.WriteLine($"Hello, World! {connectionString}");

相关文档：ASP.NET Core 6(.NET 6) Program.cs中使用读取appsettings.json配置文件