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

Use a StringBuilder instead of several million concatenations.

If you concatenate 2 strings, this means the system allocates new memory to contain both of them, and then copies both in. A zillion large memory allocations and copy actions become slow very fast.

What StringBuilder does is reduce this immensely by allocating 'in advance', thus only having to grow the buffer a few times and just copying it in, eliminating the by far slowest factor of your loop.