If you use a Sql Source to populate the check list (As shown here: http://msdn.microsoft.com/en-us/library/heyd79hc.aspx), trying to change the status of a check item may not work during a Page_Load event (for example, trying to change what is checked in your list). This is likely because the checklist has not finished databinding to its source.

What worked for me is, adding an event handler to the OnDataBound event of the check box list.
The other option is probably to call this.Databind(); or CheckBoxList2.Databind(); in the Page_Load; then, doing the check/uncheck logic. I did not try that however.
<asp:CheckBoxListID="CheckBoxList2"runat="server"DataSourceID="SqlDataSource1"DataTextField="Name"DataValueField="pkid"RepeatDirection="Horizontal"CellPadding="1"CellSpacing="3"RepeatColumns="4"OnDataBound="CheckBox2_Bound">
</asp:CheckBoxList>

Then, in your code behind, running something similar to what is below to initialize your check list with the correct selected items:

protected void CheckBox2_Bound(object sender, EventArgs e) {     
   CheckIt(thisUsr.AllowedList, CheckBoxList2); 
} 

private void CheckIt(string AllowedList, CheckBoxList CheckList) { 
  System.Diagnostics.Debug.WriteLine("Checking " + CheckList.ID); 
  for (int i = 0; i < CheckList.Items.Count; i++) { 
    System.Diagnostics.Debug.WriteLine("Checking if user has " + CheckList.Items[i].Value); 
    if (AllowedList.Contains(CheckList.Items[i].Value)) { 
      CheckList.Items[i].Selected = true; 
      System.Diagnostics.Debug.WriteLine("Yes"); 
    } else { System.Diagnostics.Debug.WriteLine("No"); } 
  } 
}

Cheers,

It seems like there is no property on Checkboxlist that would specify which item is selected "checked" and the time of Databinding.

1. The optional is to iterate though the whole checkboxlist after databoud but why would I need to go through the same list twice if I already know which item is selected at ondatabinding.

2. Event "ondatabinding" doesn't provide the access to ListItem currently bound nor the DataItem that is used which makes this event completely useless.

So why is this control so clunky, no one ever thought of "preselecting" items at databind???

As you may know the CheckBoxList renders html <input type="checkbox"> and some more (<label ...)

In the description it says that the value is stored in the view state. But what if i do not want to use the viewstate ( <asp:CheckBoxList ID="CheckBoxList1" runat="server" EnableViewState="False">)? I would expect that the code below:

     DataTable dt = CameraMakers.GetBrandNamesAsDataTable();

     CheckBoxList1.DataSource = dt;
     CheckBoxList1.DataValueField = dt.Columns["ID"].ToString();
     CheckBoxList1.DataTextField = dt.Columns["description"].ToString();
     CheckBoxList1.DataBind();

would produce something like this

<input type="checkbox" name="cb1" value="121"><label for="cb1">Sony</label>
<input type="checkbox" name="cb2" value="122"><label for="cb2">Canon</label>

But even if am willing to accept that asp.net will not use the value property of the input-tag. I expect at least to be able to access the value:

        foreach (ListItem listi in CheckBoxList1.Items)
        {
            if (listi.Selected == true)
            {
                checkBoxValues += listi.Value;
            }
        }

But this will not work either if you use CheckBoxList1.DataSource instead of writing the ListItems by Hand in the aspx file (<asp:ListItem>).

Do you agree that the behaviour of this control has lots of room for improvement (<strike>is utter nonesense right now</through>).