动态添加的ASP.NET控件未存储在ViewState中

Question

I have an application where I need to add multiple (and nested) controls to a PlaceHolder. The user enters the number of 'weeks', and my application adds a RadSplitter (using the Telerik control set), along with the relevant panes/grids for the weeks. I add these controls using code behind.

This works fine when first binding (when entering the number of weeks, and clicking Submit). But I also need to enable drag and drop functionality between the controls, which causes a postback.

It seems that the number of controls in my placeholder is always '0' on postback, so I'm guessing they are not being stored in the ViewState. Rather than have to readd these on every postback, how can I ensure my controls are stored in the ViewState?

Here's some example code:

      protected void btnSubmit_Click(object sender, EventArgs e)
        {
            if (plcSplitter.Controls.Count > 0)
                plcSplitter.Controls.Remove(plcSplitter.Controls[0]);
            var splitter = new Telerik.Web.UI.RadSplitter();
            splitter.Width = Unit.Percentage(100);
            int noOfWeeks = int.Parse(txtNoOfWeeks.Text);
            DateTime dt = new DateTime(2012, 05, 13);
            for (int i = 0; i < noOfWeeks; i++)
            {
                var range = new Common.DateRange(dt.AddDays(-6),dt);
                var pane = new Telerik.Web.UI.RadPane();
                Label lbl = new Label();
                lbl.ID = "lblText";
                lbl.Text = range.To.ToShortDateString();
                pane.Controls.Add(lbl);
                var gv = AddGrid(i);
                pane.Controls.Add(gv);
                splitter.Items.Add(pane);

                var splitLine = new Telerik.Web.UI.RadSplitBar();
            splitter.Items.Add(splitLine);

                dt = dt.AddDays(-7);
            }
            plcSplitter.Controls.Add(splitter);
            splitter.DataBind();
}

Answer 1

Controls are not stored in the viewstate, only some of control properties can be stored in viewstate. So, on postback you must create these labels again.

Move that logic to create labels from btnSubmit_Click to separate method, call that method on button click and store data needed to recreate labels somewhere (maybe session), then on postback in OnInit method check for that stored data and if there is some labels create it at that event.

Be sure to read this blog post about viewstate :

http://weblogs.asp.net/infinitiesloop/archive/2006/08/03/Truly-Understanding-Viewstate.aspx

and this about creating controls in runtime

http://weblogs.asp.net/infinitiesloop/archive/2006/08/25/TRULY-Understanding-Dynamic-Controls-_2800_Part-1_2900_.aspx

Answer 2

You can store the data necessary to build your controls in the ViewState, but the really important part is that you make sure your controls are built before you try to access them.

Here's a super basic example.

    protected void Page_Load(object sender, EventArgs e)
    {
        BuildControl(GetLabelData());
    }

    private Tuple<string, string> GetLabelData()
    {
        if (Page.IsPostBack)
            return (Tuple<string, string>)ViewState["MyLabelData"];
        else
            return new Tuple<string, string>("lblTest", "Test");
    }

    private void BuildControl(Tuple<string, string> t)
    {
        Label l = new Label();
        l.ID = t.Item1;
        l.Text = t.Item2;
        ViewState["MyLabelData"] = t;
        plcSplitter.Controls.Add(l);
    }

    protected void bDoSomething_Click(object sender, EventArgs e)
    {
        Response.Write(String.Format("plcSplitter.Controls.Count:{0}", plcSplitter.Controls.Count));
    }

It's also very important to recognize that these controls are being built at the server and if they can be altered by the client you'll need to implement some mechanism of communication for the important bits of info so you can rebuild your controls and then apply any modifications from the client.

For example you are implementing a draggable control, on the client side when you drag you'll need to store the coordinates in a hidden control manually so that can be posted back to the server and you can have that info available when you're rebuilding the controls.

Answer 3

I think the reason those controls are not stored in the ViewState is that they are being added to the page "too late", after the page ViewState is generated. Look at this post:

Last event in page that can still affect a page's viewstate