这是因为你的首页动作和详细信息行动并不在同一个控制器实例执行。你可以把数据库名在会话变量并用它来创建的DbContext 实例。

That is because your Index action and Details action do not execute on the same controller instance. You can keep the database name in a session variable and use that to create the DbContext instance.

public class UsersController : Controller
{
   private UsersContext db;

   protected override void Initialize(RequestContext requestContext)
   {
        base.Initialize(requestContext);

       if (Session["Database"] == null)
          db = new UsersContext("dbA");
       else
          db = new UsersContext((string)Session["Database"]);
   }

   public ViewResult Index()
   {
       if (...)       {
          db = new UsersContext("dbA");
          Session
       else
       {
          db = new UsersContext("dbB");
          Session["Database"] = "dbB";
       }

       return View(db.Users.ToList());
   }
}