Question

I’m getting out of memory exceptions from the following function when RowCollection is 50000+ and thus i need to make it more memory efficient. The function is simply needs to construct a comma separated string of the row indexes stored in RowCollection. Can anyone spot any obvious memory hungry operations in the following?

N.B RowCollection just contains a list of row indexes stored as integers.

 Private Function GetCommaSeparatedString(ByRef RowIndexes As ArrayList) As String
        Dim RowString As String = String.Empty

        'Build a string of the row indexes 
        'Add one onto each index value so our indexes begin at 1 
        For Each Row In RowIndexes 
            RowString += CInt(Row.ToString) + 1 & ","
        Next

        'Remove the last comma
        If RowString.Length > 0 Then
            RowString = RowString.Substring(0, RowString.Length - 1)
        End If

        Return RowString
    End Function

Thanks in advance.

Answer 1

I’m not sure why you’re getting out of memory errors, unless the string representation of your rows is extremely large, because you never have more than one or two non-garbage-collectible strings.

However, your method is horribly inefficient because it spends so much time copying the contents of half-built strings. A StringBuilder is more appropriate when building large strings, because it can be modified without re-creating the contents each time.

HOWEVER, in this case even a StringBuilder is a bad idea, because you are joining strings and there is already a method to do that: String.Join. Just use a LINQ query to do the add-one-to-index-stuff and you get a one-liner:

Private Function GetCommaSeparatedString(ByVal RowIndexes As ArrayList) As String
    Return String.Join(",", From index In RowIndexes Select CInt(index) + 1)
End Function

I would also recommend not passing by reference unless you actually need it. You aren’t modifying RowIndexes, so pass it by value. I’m also not sure why you are ToString()-ing the index then immediately parsing it. Aren’t they already integers? Just use CInt.