Most efficient way to concatenate strings?

Question

What\'s the most efficient way to concatenate strings?

Answer 1

The StringBuilder.Append() method is much better than using the + operator. But I've found that, when executing 1000 concatenations or less, String.Join() is even more efficient than StringBuilder.

StringBuilder sb = new StringBuilder();
sb.Append(someString);

The only problem with String.Join is that you have to concatenate the strings with a common delimiter.

Edit: as @ryanversaw pointed out, you can make the delimiter string.Empty.

string key = String.Join("_", new String[] 
{ "Customers_Contacts", customerID, database, SessionID });