StringBuilder query = new StringBuilder();
                    query.Append("CREATE TABLE #Codes (Code nvarchar(100) collate database_default ) ");
                    query.Append("Insert into #Codes (Code) ");
                    int lengthOfCodesArray = targetCodes.Length;
                    for (int index = 0; index < lengthOfCodesArray; index++)
                    {
                        string targetCode = targetCodes[index];
                        query.Append("Select N'" + targetCode + "' ");
                        if (index != lengthOfCodesArray - 1)
                        {
                            query.Append("Union All ");
                        }
                    }
  query.Append("drop table #Codes ");

on: cmd.ExecuteReader() I get

There is insufficient system memory to run this query when creating temporary table

But weird thing is that, when I have 25k codes is ok, when 5k I get this error. 

Initial size is 262 MB.

Lengt of each code is average 15.