>Given the following HTML fragment, how can I clear the checkmark, in the checkbox with id = "checkAll", in code behind? I can get e reference to the containing grid with:
>
>

DirectCast(Page.FindControl("dgListItems"), DataGrid)

>
>but I can't figure out how to get a reference to the checkbox in the HeaderTemplate in the ColumnTemplate.
>

If you're doing this from an event of the checkbox, you can just use:

CheckBox check = sender as CheckBox;
if (check != null)
   check.Checked = true;

If it's from some other event handler, or just in something like a normal postback, you can't do this directly. Actually, I don't think they expose a FindControl for that kind of thing (at least, I didn't see one). You could do this yourself though by traversing the Controls collection of the grid:

CheckBox check = this.FindGridControl(this.grdSample.Controls, "checkAll") as CheckBox;
if (check != null)
    check.Checked = true;

protected Control FindGridControl(ControlCollection control, string controlName)
{
    Control matchingControl = null;

    foreach (Control subControl in control)
    {
        if (subControl.ClientID.EndsWith(controlName))
            return subControl;

        matchingControl = this.FindGridControl(subControl.Controls, controlName);
        if (matchingControl != null)
            break;
    }

    return matchingControl;
}