The regular “Thread Safety” section of the MSDN documentation for StringBuilder states that “any instance members are not guaranteed to be thread safe” but this statement feels like it has been copied and pasted for almost every class in the Framework:
http://msdn.microsoft.com/en-us/library/system.text.stringbuilder.aspx
However, these blog posts by Gavin Pugh mention thread-safe behaviours of StringBuilder:
http://www.gavpugh.com/2010/03/23/xnac-stringbuilder-to-string-with-no-garbage/
http://www.gavpugh.com/2010/04/01/xnac-avoiding-garbage-when-working-with-stringbuilder/
Additionally, the source of StringBuilder revealed by Reflector, and the accompanying comments
in the SSCLI source, also suggest many implementation considerations to ensure thread-safety:
Does anyone have any more insight into whether a StringBuilder instance is safe to share among multiple concurrent threads?