然后重点在是 Startup.cs文件里添加 services.AddSingleton<接口名, 实例化名>();
public void ConfigureServices(IServiceCollection services)
{
services.AddSingleton<Ibook, ChineseBook>();
//services.AddSingleton<Ibook, EnglishBook>(); //同名只会取值最后面的
services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}