Performance issues with nested loops and string concatenations

Question

Can someone please explain why this code is taking so long to run (i.e. >24 hours): The number of rows is 5000, whilst the number of columns is 2000 (i.e. Approximately 10m loop

Answer 1

Your code is taking so long because you're appending strings, creating thousands of new temporary strings as you go. The memory manager needs to find memory for these strings (which increase in memory requirements, as they get longer) and the operation copies the characters you have so far (the number of which increases with every iteration) to the newest string.

The alternative is to use a single StringBuilder, on which you call Append() to append more efficiently and, finally, ToString() when you're done to get the finalized string that you want to use.