What\'s the best string concatenation method using C#?

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

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

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

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