string DateEndLast ="'"+ txtDateEndLast.Value+"'";
if (txtDateEndLast.Value.Trim() == "")
{
DateEndLast = "null";
}

string sql = "update DB_****t set style='" + style + "',ClientID='" + ClientID + "',Title='" + Title + "',DateStart='" + DateStart + "',DateEnd='" + DateEnd + "',DateEndLast=" + DateEndLast + "   where ;

执行语句 时  进行替换 即可保存。