Instead of handling OnClick event for the button, handle ItemCommand event
for the datalist:<asp:DataList
OnItemCommand="Item_Command"><ItemTemplate><asp:Button CommandName="XXX"
/></ItemTemplate></asp:DataList>

and in the code behind:
void Item_Command(Object sender, DataListCommandEventArgs e)
{
       if (e.CommandName == "XXX")
           string label = (e.Item.FindControl("labelD") as Label).Text;
}