This is a common task that I searched high and low for but ended up coding along with some pieces from here and there.  All I wanted was to have a group of checkboxes with one being a "select all" that would, naturally, select all others in its group when selected and un-select them when it was un-selected.  My first pass was to use a checkbox list since they were going to be bound to an ArrayList and then have one stand alone check box to use for "all" that would fire an event to iterate through the others.  That required a trip to the server plus some groups have a built-in selection that is equivalent to "all"...that left me with the chance to have two "all" checkboxes.

So I decided to use a DataList and a client-side function.  I have one HtmlInputCheckbox in the DataList and I use the OnItemDataBound event and add an "OnClick" attribute for box that represent "all" for that group.

Here's the DataList in the aspx page.  I'm using the field from the ArrayList as both the value and display text here.

<asp:DataList ID="dataList" OnItemDataBound="DataList_ItemDataBound" Runat=server>
	<ItemTemplate >
		<input type=checkbox id="chkBox" name="chkBox" runat=server
                  value='<%# Container.DataItem %>'><%# Container.DataItem %>
	</ItemTemplate>
</asp:DataList>

protected void DataList_ItemDataBound(object sender, DataListItemEventArgs e)
{
	if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
	{
		HtmlInputCheckBox chkData = (HtmlInputCheckBox)e.Item.FindControl(chkBox);
		if(chkData.Value.ToLower() == "all")
		{
		   chkData.Attributes.Add("onclick", "CheckAll(this,'" + chkData.Name + "');");
		}
	}
}

<script language=JavaScript>
function CheckAll(checkAllBox, fieldName)
{
	var frm = document.Form1;
	var actVar = checkAllBox.checked;
	for(i = 0; i < frm.length; i++)
	{
		e = frm.elements[i];
		if (e.type == 'checkbox' && e.name.indexOf(fieldName) != -1)
			e.checked = actVar ;
	}
}
</script>