.Net 5，appsettings.json在Program.cs中不可用？

var config = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json")
    .Build();

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration((hostingContext, config) =>
        {
            config.AddConfiguration(configuration);
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        });

public class MyClass
{
    private readonly IConfiguration _configuration;

    public MyClass(IConfiguration configuration)
    {
        _configuration = configuration;
    }

    public void MyMethod()
    {
        var value = _configuration["MyConfigKey"];
        // 使用配置信息进行操作
    }
}