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LINQ Convert string to datetime
DateMonth=convert(datetime, convert(char(6), AASI.Inv_Acctcur) + '01')

"yyyyMMdd"

SELECT convert(datetime, '20161023', 112) -- yyyymmdd

// Doing .ToArray is essential, to ensure that the
// query is executed right away.
string[] inMemoryCollectionStrings = (
    from item in db.Items
    where some_condition
    select item.Inv_Acctcur).ToArray();

// This next query does the translation from string to DateTime.
IEnumerable<DateTime> dates =
    from value in inMemoryCollectionStrings
    select DateTime.ParseExact(value, "yyyyMMdd",
        CultureInfo.InvariantCulture);

DateTime.ParseExact(value, "yyyyMMdd", CultureInfo.InvariantCulture);