Simple question: How to cache a List<> in ASP.Net?

I'd do this with the ASP.NET Cache API - here's how.

Imports System.Web

Public Class CacheManager

Private ListKey As String = "MyList"

Public Shared ReadOnly Property TypedList As List(Of Integer)

    Dim cachedObject As Object  
    Dim myList As List (Of Integer)  
    Dim userCacheKey As String = ListKey & HttpContext.Current.User.Identity.Name

    'First check to see if List is in the cache already
    cachedObject = HttpRuntime.Cache.Get(userCacheKey)

    If cachedObject Is Nothing Then
       'If List isn't in the cache already then get it...
       myList = Code to retrieve list members goes here
       ' ...and now we've got it put it in the cache
       HttpRuntime.Cache..Add(key:=userCacheKey, value:=myList, absoluteExpiration:=HttpRuntime.Cache.NoAbsoluteExpiration, slidingExpiration:=New TimeSpan(0,5,0), dependencies:=Nothing, onRemoveCallback:=Nothing, priority:=CacheItemPriority.Default)
    Else
        'List is already in the cache but everything comes out of the cache as System.Object so cast it to List (Of Integer)  
        myList = DirectCast(cachedObject, List (Of Integer))
    End If

    'Now we have List, return it to the caller
    Return myList

End Property

End Class

This gives us a class that will hold an instance of List<> per user that exists in memory for five minutes after the last time it was accessed - you can up this just by changing the length of the TimeSpan object in the slidingExpiration parameter when the List is added to the Cache.

Your usage in the page is then simply:

Public Sub Page_Load (arguments)

Dim myList As List(Of Integer)  
...
myList = CacheManager.TypedList
...
End Sub

<WebMethod()> Public Sub MyEventMethod(arguments)

Dim myList As List(Of Integer)
...
myList = CacheManager.TypedList
...

End Sub

It's not quite clear (to me) from your question whether users can change their individual List or they change the global list. If they change their individual list that's easy to cater for - change the TypedList property like this:

Imports System.Web

Public Class CacheManager

Private ListKey As String = "MyList"

Public Shared Property TypedList As List(Of Integer)
Get
    Dim cachedObject As Object  
    Dim myList As List (Of Integer)  
    Dim userCacheKey As String = ListKey & HttpContext.Current.User.Identity.Name

    'First check to see if List is in the cache already
    cachedObject = HttpRuntime.Cache.Get(userCacheKey)

    If cachedObject Is Nothing Then
       'If List isn't in the cache already then get it...
       myList = Code to retrieve list members goes here
       ' ...and now we've got it put it in the cache
       HttpRuntime.Cache.Add(key:=userCacheKey, value:=myList, absoluteExpiration:=HttpRuntime.Cache.NoAbsoluteExpiration, slidingExpiration:=New TimeSpan(0,5,0), dependencies:=Nothing, onRemoveCallback:=Nothing, priority:=CacheItemPriority.Default)
    Else
        'List is already in the cache but everything comes out of the cache as System.Object so cast it to List (Of Integer)  
        myList = DirectCast(cachedObject, List (Of Integer))
    End If

    'Now we have List, return it to the caller
    Return myList
End Get
Set (ByVal value As List(Of Integer))

    Dim userCacheKey As String = ListKey & HttpContext.Current.User.Identity.Name

    HttpRuntime.Cache.Insert(key:=userCacheKey, value:=value, absoluteExpiration:=HttpRuntime.Cache.NoAbsoluteExpiration, slidingExpiration:=New TimeSpan(0,5,0), dependencies:=Nothing, onRemoveCallback:=Nothing, priority:=CacheItemPriority.Default)

End Set
End Property

End Class

If any user making changes to the list changes it for everybody, then I'd look at using a CacheDependency.

I suppose that you want to cache (or rather persist) it separately for the user, as you mention that there can be changes.

Store the list in a Session variable, and enable session state for the web method using the [WebMethod(EnableSession:=True)] attribute.

Consider the amount of memory that you are using, though. Putting the list in a session variable will keep it in memory for a long time. (20 minutes is eons in the scope of a web application, where objects normally survive only a few milliseconds...)

You could make a property which checks if the session field contains data, if not load the data, otherwise return the field. Like so:

List<goof> data {
    get {
        List<goof> data = Session["data"] as List<goof>;
        if (data == null) Session["data"] = LINQstuff;

        return data;
    }
}

I suggest to use the System.Web.Caching.Cache class.

See http://msdn.microsoft.com/en-us/library/system.web.caching.cache.aspx

It doesn't depend on the user session but in your case I think it is better because you are inside a WebMethod.

Davide

[WebMethod(EnableSession = true)]
public void SetCache()
{
    LoginData userList = new LoginData
    {
        Status = "Success",
        LastLogin = DateTime.Now.ToString("yyyyMMdd-HHMMss"),
        UserName = "Some User"
    };
    HttpRuntime.Cache["loggedIn"] = userList;
}
public class LoginData
{
    public string Status { get; set; }
    public string UserName { get; set; }
    public string LastLogin { get; set; }
}

There is a big difference between a cached list and a session list and different roles they play.