您可以使用条件编译：

public Startup(IHostingEnvironment env)
{
    var builder = new ConfigurationBuilder()
    .SetBasePath(env.ContentRootPath)
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
#if SOME_BUILD_FLAG_A
    .AddJsonFile($"appsettings.flag_a.json", optional: true)
#else
    .AddJsonFile($"appsettings.no_flag_a.json", optional: true)
#endif
    .AddEnvironmentVariables();
    this.configuration = builder.Build();
}