ViewState Backed Property not working for list <T>

Question

I am trying to save a generic list in the 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, UploadedFiles.Count stays zero:

var uploadedFile = new UploadedFile {FileName = args.FileName, FileGuid = args.FileGuid};
UploadedFiles.Add(uploadedFile); // UploadedFiles.Count == 0 here!

Anyone have an idea?

Answer 1

When you create the original list, you do not save it in 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 are actually doing here is to get a property (which will initially return a new list) and then add something to this new list, I think this is actually what you want to do:

List<UploadedFile> list = UploadedFiles;
list.Add(uploadedFile);
UploadedFiles = list;

This will write to the ViewState after changing the list.

Answer 3

, - , ViewState . - , .

// 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 p>

    set

    {

        ViewState.Add("Lista", value);

    }

    get

    {

        return ViewState["Lista"] != null ? (List<UploadedFile>)ViewState["Lista"] : null;

    }

}