Question

While writing code for connecting two webparts,I created a two labels. the text value for both the labels were written in OnPreRender method. However, i forgot to add control for one label in CreateChildControl method.
So while debugging, i noticed that, after post back, the label whose control i forgot to add didn’t retain the value and it was showing empty string.But the other label who’s control i added was able to retain the value.

protected override void CreateChildControls()
    {
        base.CreateChildControls();




        btnup.Text = "  Update";

        this.Controls.Add(lblid);//**If i add this, the label retains the value during post back , otherwise its null**


        this.Controls.Add(lblname);
        this.Controls.Add(lbldesig);
        this.Controls.Add(tbdes);
        this.Controls.Add(lblcomp);
        this.Controls.Add(tbcomp);
        this.Controls.Add(btnup);

        btnup.Click += new EventHandler(btnup_Click);
    }


protected override void OnPreRender(EventArgs e)
    {
        base.OnPreRender(e);

        if (connectionInterface != null)
        {

            id = connectionInterface.parameter1;

            SPWeb mysite = SPContext.Current.Web;
            SPList mylist = mysite.Lists["UpdateList"];
            SPListItemCollection itemcol = mylist.Items;

            foreach (SPListItem itm in itemcol)
            {
                string nm = itm["Company_Id"].ToString();
                if (nm.Equals(id))
                {
                    lblid.Text = itm["Company_Id"].ToString();
                    lblname.Text = itm["Name"].ToString();
                    l
                }

            }


        }
        else
        {
            lblname.Text = "nothing is recieved!";
        }

    }

Why is it behaving like this?

Answer 1

This is a normal behavior. If you don’t add the control to Controls collection, ASP.NET framework will not preserve its value in postback and hence will be lost during postback.