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copied!<p>Short answer: it depends.</p> <p>Long answer: <strong>if you already have an array of strings to concatenate together (with a delimiter), <code>String.Join</code> is the fastest way of doing it.</strong></p> <p><code>String.Join</code> can look through all of the strings to work out the exact length it needs, then go again and copy all the data. This means there will be <em>no</em> extra copying involved. The <em>only</em> downside is that it has to go through the strings twice, which means potentially blowing the memory cache more times than necessary.</p> <p>If you <em>don't</em> have the strings as an array beforehand, it's <em>probably</em> faster to use <code>StringBuilder</code> - but there will be situations where it isn't. If using a <code>StringBuilder</code> means doing lots and lots of copies, then building an array and then calling <code>String.Join</code> may well be faster.</p> <p>EDIT: This is in terms of a single call to <code>String.Join</code> vs a bunch of calls to <code>StringBuilder.Append</code>. In the original question, we had two different levels of <code>String.Join</code> calls, so each of the nested calls would have created an intermediate string. In other words, it's even more complex and harder to guess about. I would be surprised to see either way "win" significantly (in complexity terms) with typical data.</p> <p>EDIT: When I'm at home, I'll write up a benchmark which is as painful as possibly for <code>StringBuilder</code>. Basically if you have an array where each element is about twice the size of the previous one, and you get it just right, you should be able to force a copy for every append (of elements, not of the delimiter, although that needs to be taken into account too). At that point it's nearly as bad as simple string concatenation - but <code>String.Join</code> will have no problems.</p>

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