Which for loop is faster

Solution 1 by _Asif_[^] is very good, even if does not contain exact answer.

Traditional for loop is faster than Parrarel loop and even Linq method too:

List<string> Days = Enumerable.Range(1,31).Select(x=>x.ToString("00")).ToList();

Time of execution:

1) traditional:  00:00.070 s
2) parrarel:     00:00.197 s
3) linq:         00:00.330 s

The loop can't be parallelized to start with, since Days.Add() requires sequential processing. Therefore parallel.for just adds overhead without improving performance.

The key thing to look for is whether there is a variable that gets modified with each execution.

In this case, Days is changed with each execution, and mixing the order of execution will mix up the order of elements in the final list.

P.S.: I just realized this assessment isn't entirely correct. I've only considered the command Days.Add() without considering the command embedded with the function parameter: i.ToString(). The later can be parallelized, as it doesn't depend on the order of processing. However, if you don't immediately store the string in the list, the overhead of creating an additional temporary string probably outweighs the benefit of parallelizing the string conversion. So it isn't as clear cut as I outlined it above.