Rico Mariani, the .NET Performance guru, had an article on this very subject. It’s not as simple as one might suspect. The basic advice is this:
If your pattern looks like:
x = f1(...) + f2(...) + f3(...) + f4(...)
that’s one concat and it’s zippy, StringBuilder probably won’t help.
If your pattern looks like:
if (...) x += f1(...)
if (...) x += f2(...)
if (...) x += f3(...)
if (...) x += f4(...)
then you probably want StringBuilder.
Yet another article to support this claim comes from Eric Lippert where he describes the optimizations performed on one line +
concatenations in a detailed manner.