{
	"Data": 
	{
		"DefaultConnection": 
		{
			"ConnectionString": "Server=(localdb)\\MSSQLLocalDB;Database=Books;Trusted_Connection=True;"
		}
	},
	"Appsettings": 
	{
 "Name": "XiaoLi",
 "Date": "2020-04-18",
 "age": "20"
 },
 "url":"https://blog.csdn.net/weixin_42686768"
}

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Configuration.Json;
using System.IO;

public static string GetConnectionString(string value)
{
 var configurationBuilder = new ConfigurationBuilder()
 .SetBasePath(Directory.GetCurrentDirectory())
 .AddJsonFile("config.json");

 //var configurationBuilder = new ConfigurationBuilder()
 // .Add(new JsonConfigurationSource { Path = "config.json", ReloadOnChange = true });

 IConfiguration config = configurationBuilder.Build();
 string connectionString = config[value];
 return connectionString;
}

// 1、獲取 ConnectionString 對應的字符串
va = "Data:DefaultConnection:ConnectionString";
// 2、獲取 Name 對應的字符串
va = "Appsettings:Name";
// 3、獲取 url 對應的字符串
va = "url";

GetConnectionString(va)