.net - At what point does using a StringBuilder become insignificant or an overhead?

Question

Recently I have found myself using StringBuilder for all string concatenations, big and small, however in a recent performance test I swapped out a colleague's stringOut = string1 + "." string2 style concatenation (being used in a 10000x + loop with the StringBuilder being newed each time) for a StringBuilder just to see what difference it would make in a minor concatenation.

I found, over many runs of the performance test, the change was both insignificantly higher or lower regardless of concatenation or StringBuilder (restating this was for small concatenations).

At what point does the 'newing up' of a StringBuilder object negate the benefits of using one?

Answer 1

The rule that I follow is -

Use a StringBuilder when the number of concatenations is unknown at compile time.

So, in your case each StringBuilder was only appending a few times and then being discarded. That isn't really the same as something like

string s = String.Empty;
for (int i = 0; i < 10000; ++i)
{
    s += "A";
}

Where using a StringBuilder would drastically improve performance because you would otherwise be constantly allocating new memory.