LINQ SELECT中 string 和datatime 格式转换出错

   
             var RelfnoAsso = from reliefnoasso in
                                (from relief in dsGLVRELF.Tables[0].AsEnumerable()
                                 join associated in dsLKMAsso.Tables[0].AsEnumerable() on relief.Field<string>("SERVICE_TAG") equals associated.Field<string>("SERVICE_TAG") into temp
                                 from xx in temp.DefaultIfEmpty()
                                 //where associated.Field<string>("SERVICE_TAG") == null
                                 select new
                                 {
                                     SERVICE_TAG=relief.Field<string>("SERVICE_TAG"),
                                     TRANSACTION_TYPE = relief.Field<string>("transaction_type"),
                                     GLV_TRANSACTION_DATE= relief.Field<DateTime>("transaction_date"),
                                     ITEM= relief.Field<string>("item"),
                                     ITEM_QTY= relief.Field<double>("item_qty"),
//下面这一句，我怎么改都不对，改成string也提示错误，datatime也提示错误，前面加system.Datatime也提示错误，请大神指教
                                     LKM_TRANSACTION_DATE=(xx==null)?"":xx.Field<DateTime>("LAST_UPDATE_DATE")                                 }
                                )
                                where string.IsNullOrEmpty(reliefnoasso.LKM_TRANSACTION_DATE)
                                 select new
                                 {
                                     SERVICE_TAG = reliefnoasso.SERVICE_TAG,
                                     TRANSACTION_TYPE = reliefnoasso.TRANSACTION_TYPE,
                                     GLV_TRANSACTION_DATE = reliefnoasso.GLV_TRANSACTION_DATE,
                                     ITEM = reliefnoasso.ITEM,
                                     ITEM_QTY = reliefnoasso.ITEM_QTY,
                                     LKM_TRANSACTION_DATE = reliefnoasso.LKM_TRANSACTION_DATE
                                 };
错误 CS0173 无法确定条件表达式的类型，因为“string”和“System.DateTime”之间没有隐式转换