Concatenation with StringBuilder

Question

Is it from a sight of resources reasonable to use a StringBuilder already to concat two strings or is there a minimum concatenation operations that makes the StringBuilder efficient?

Answer 1

...the String class is preferable for a concatenation operation if a fixed number of String objects are concatenated

Answer 2

String.Join is the fastest one as it allocates all the required memory in one operation. See this article: StringBuilder vs. String / Fast String Operations with .NET 2.0

Answer 3

Concatenating two strings with StringBuilder won't give you any benefits, since the result still has to be converted to a string - so concatenating them directly is one allocation + two copies. StringBuilder can't do any better - only clutter your code.