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

appsettings.json内容

{
   "ConnectionString": "Server=localhost;Database=tempDB;Uid=<dbUserName>;Pwd=<dbPassword>"
}

2.1、使用ConfigurationBuilder读取appsettings.json

实现代码

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

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

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>("ConnectionString");
//另一种方法
string connectionString = config["ConnectionString"];