我尝试了以下左联接查询：

var ResultData = (from rowCurrent in currentDay.AsEnumerable()
            join rowPrevious in previousDay.AsEnumberable()
            on new { ID = rowCurrent.Field<string>("ID"), Account = rowCurrent.Field<string>("Account") }
            equals new { ID = rowPrevious .Field<string>("ID"), Account = rowPrevious .Field<string>("Account") }
            into rowRight
            from rowJoin in rowRight.DefaultIfEmpty()
            where
            rowJoin == null ?
            (
                (rowCurrent.Field<string>("IsEmail") == "Y") &&
                (IsValidEmail(rowCurrent.Field<string>("EmailAddress")))
            ) :
            (
                (rowCurrent.Field<string>("IsEmail") != rowJoin?.Field<string>("IsEmail")) ||
                (rowCurrent.Field<string>("EmailAddress") != rowJoin?.Field<string>("EmailAddress"))
            )
            select new ResultData
            {
                 ID = rowCurrent.Field<string>("ID"),
                 Account = rowCurrent.Field<string>("Account"),
                 IsEmail = rowCurrent.Field<string>("IsEmail"),
                 EmailAddress = rowCurrent.Field<string>("EmailAddress")
            }
).ToList();

--如果发现不匹配的行，则仅当它是有效记录时才选择。
或其他
--如果找到匹配行，则将选择“IsEmail”或“EmailAddress”值是否已从PreviousDay更改