String vs. StringBuilder

Yes, the performance difference is significant. See the KB article "How to improve string concatenation performance in Visual C#".

I have always tried to code for clarity first, and then optimize for performance later. That's much easier than doing it the other way around! However, having seen the enormous performance difference in my applications between the two, I now think about it a little more carefully.

Luckily, it's relatively straightforward to run performance analysis on your code to see where you're spending the time, and then to modify it to use StringBuilder where needed.

To clarify what Gillian said about 4 string, if you have something like this:

string a,b,c,d;
 a = b + c + d;

then it would be faster using strings and the plus operator. This is because (like Java, as Eric points out), it internally uses StringBuilder automatically (Actually, it uses a primitive that StringBuilder also uses)

However, if what you are doing is closer to:

string a,b,c,d;
 a = a + b;
 a = a + c;
 a = a + d;

Then you need to explicitly use a StringBuilder. .Net doesn't automatically create a StringBuilder here, because it would be pointless. At the end of each line, "a" has to be an (immutable) string, so it would have to create and dispose a StringBuilder on each line. For speed, you'd need to use the same StringBuilder until you're done building:

string a,b,c,d;
StringBuilder e = new StringBuilder();
 e.Append(b);
 e.Append(c);
 e.Append(d);
 a = e.ToString();