ViewState Backed Property not Working for List<T> - asp.net

Question

I am trying to store a generic list in a viewstate-backed property as follows:
[Serializable]
public class UploadedFile
{
public string FileName { get; set; }
public Guid FileGuid { get; set; }
}
public List<UploadedFile> UploadedFiles
{
get
{
return (List<UploadedFile>) (ViewState["UploadedFiles"] ?? new List<UploadedFile>());
}
set
{
ViewState["UploadedFiles"] = value;
}
}
When I try to add an item to the list, the UploadedFiles.Count remains zero:
var uploadedFile = new UploadedFile {FileName = args.FileName, FileGuid = args.FileGuid};
UploadedFiles.Add(uploadedFile); // UploadedFiles.Count == 0 here!
Anyone have an idea here?

Answer 1

When you create the initial list, you dont save it into viewstate, try this..
public List<UploadedFile> UploadedFiles
{
get
{
var list = (List<UploadedFile>) (ViewState["UploadedFiles"] ??
new List<UploadedFile>());
ViewState["UploadedFiles"] = list;
return list;
}
set
{
ViewState["UploadedFiles"] = value;
}
}

Answer 2

What you're actually doing here is getting the property (which will initially return a new List) and then adding something to that new list, I think this is actually what you want to do:
List<UploadedFile> list = UploadedFiles;
list.Add(uploadedFile);
UploadedFiles = list;
This will write back to the ViewState after modifying the list.

Answer 3

To make life easier, I usually just do something like this to track ViewState for objects. Then you don't need any special handling when you refer to it elsewhere in code.
// Lazy loading object
protected List<string> ItemList {
get {
if (_ItemList==null) {
_ItemList = new List<string>();
}
return(_ItemList);
}
}
protected list<string> _ItemList=null;
// Save & Load it to viewstate as needed
protected override object SaveViewState()
{
if (_ItemList != null)
{
ViewState["ItemList"] = ItemList;
}
return base.SaveViewState();
}
protected override void LoadViewState(object savedState)
{
base.LoadViewState(savedState);
if (ViewState["ItemList"] != null)
{
_ItemList = (List<string>)ViewState["ItemList"];
}
}

Answer 4

private List Lista
set
{
ViewState.Add("Lista", value);
}
get
{
return ViewState["Lista"] != null ? (List<UploadedFile>)ViewState["Lista"] : null;
}
}