How to persist List<T> property in ASP.NET Custom Control?

Question

I have the following property in a custom control:

List<myClass> _items;
public List<myClass> Items{
    get { return _items; }
    set { _items= value; }
}

In my codebehind, I add items to the collection as in...

myCustomControl.items.Add(new myClass());

However, these are not persisted across postbacks. What is the proper way to allow persistance in custom controls?

Answer 1

if you are talking about persisting the data across postbacks of the same page then you can manually add the items to the ViewState and the retrieve them On Load.

Answer 2

You can store them in the controls viewstate

public List<myClass> Items{
    get { return this.ViewState["itemsKey"] }
    set { this.ViewState["itemsKey"]= value; }
}

Answer 3

Yikes! Don't put a List<> into the ViewState! It'll be massive!

If you add a List<string> that contains two elements - "abc" and "xyz" into the ViewState, it'll grow by 312 bytes.

If instead you add a string[] that contains the same two elements, it'll only grow by 24 bytes.

And that's just lists of strings! You can put your classes in there as Corey Downie suggests, but your ViewState will mushroom!

To keep it a sensible size, you'll have to go to some effort to convert your list of items into arrays of strings and back again.

As an alternative, consider putting your objects into the Session: that way your objects will be stored on the server, instead of being serialized into the ViewState and sent to the browser and back.

Answer 4

I agree that there are issues with a List<> in viewstate but it does work. Note that there is no setter on this by design. You need a reference to the list object object and your get method can new up a list when needed.

protected List<myClass> Items
{
    get
    {
        if (ViewState["myClass"] == null)
            ViewState["myClass"] = new List<myClass>();

        return (List<myClass>)ViewState["myClass"];
    }
}

Answer 5

One way to overcome the size problem with a generic list is to persist it in ViewState as its basic array type:

protected List<string> Items 
{ 
    get 
    { 
        if (ViewState["Items"] == null)
            ViewState["Items"] = new string[0];
        return new List<string>((string[])ViewState["Items"]);
    }
    set
    {
        ViewState["Items"] = value.ToArray();
    }
}