Well as with all things in programming there are almost no definites but in general if you are building a long string from many parts then using the StringBuilder class is better.
If you've got a particular instance where you can't decide then post it here although performance measuring is hard, you need to know in what circumstances the code is run, web or desktop, multiple users or one at a time, processor count, threading etc.
Also String has many methods that StringBuilder doesn't so you don't always have a choice.
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Joe (
Microsoft MVP - XML)