将 object 存储在 session 变量中

Question

I have a dropdown menu that when you select an option value submit the form, and to avoid repetitive database calls I am storing my non-sensitive object in a session.

private List<Employee> stafflist
{
    get { return Session["stafflist"] as List<Employee>; }
    set { Session["stafflist"] = new Employee(); }
}
private void RemoveStaff()
{
    Session.Remove("stafflist");
}

however in my

[HttpPost]
public ActionResult index (...)
{
    //why can't I get the list of staff like this?
    ViewBag.staff=stafflist.Where(..).toList();

    //is the below still needed? i thought i 
    //have a session variable declare above, 
    //and to avoid 30x repetitive db calls? 
    //also note when i include the below the code runs fine, 
    //however, if i take it out it doesn't. i would like to avoid repetitive db calls
    stafflist=db.Employee.toList(); 
}

Answer 1

First of all, you should not prevent to query the database. Proper caching is hard to get right, and a database is perfectly capable of performing queries and caching data.

If you're absolutely sure you want to circumvent the database, and query clientside (ie in the controller) then you need to pull the entire staff list from the database at least once per visitor.

You could do that in the first GET call to this controller, assuming the user will always visit that:

[HttpGet]
public ActionResult Index (...)
{
    var cachedStaff = db.Employee.toList(); 
    Session["stafflist"] = cachedStaff;
}

Then in the POST, where you actually want to do the database query (again, consider letting the database do what it's good at), you can query the list from the session:

[HttpPost]
public ActionResult Index (...)
{
    var cachedStaff = Session["stafflist"] as List<Employee>();

    // TODO: check cachedStaff for null, for when someone posts after 
    // their session expires or didn't visit the Index page first.

    var selectedStaff = cachedStaff.Where(..).ToList();

    // the rest of your code
}

Then the property you introduced can be used as syntactic sugar to clean up the code a bit:

private List<Employee> CachedStaff
{
    get { return Session["stafflist"] as List<Employee>; }
    set { Session["stafflist"] = value; }
}

[HttpGet]
public ActionResult Index (...)
{
    CachedStaff = db.Employee.toList(); 
}

[HttpPost]
public ActionResult Index (...)
{
    // TODO: this will throw an ArgumentNullException when
    //  the staff list is not cached, see above.
    var selectedStaff = CachedStaff.Where(..).ToList();

    // the rest of your code
}

Answer 2

A session is unique for the current user and the current session. That means that when the user closes the browser, the session information is lost. The session is also lost if the session cookie is removed. Read about state management .

If you want to have a global staff list that is available for all users you need to use something else. Caching is the most common case then.

Answer 3

you probably have it already figured it out, just in case I leave here what it worked for me.

First you create a new session variable based on an object created (in this case the object usr will be empty):

User usr = new User();
Session["CurrentUSR"]=usr;

where you want to use the new object, you will have to cast the session variable and point it to a new object created in that particular page:

User usr= new User(); //at this point the usr object is empty, now you are going to replace this new empty object with the session variable created before
usr=Session["CurrentUSR"] as User();

In case you have a list, the best course of action would be to create a List<> of that particular object.