Assemblies: mscorlib (in mscorlib.dll)
System.IO (in System.IO.dll)
Flushing the stream will not flush its underlying encoder unless you explicitly call Flush or Close. Setting to true means that data will be flushed from the buffer to the stream after each write operation, but the encoder state will not be flushed. This allows the encoder to keep its state (partial characters) so that it can encode the next block of characters correctly. This scenario affects UTF8 and UTF7 where certain characters can only be encoded after the encoder receives the adjacent character or characters.
When AutoFlush is set to false, StreamWriter will do a limited amount of buffering, both internally and potentially in the encoder from the encoding you passed in. You can get better performance by setting AutoFlush to false, assuming that you always call Close (or at least Flush) when you're done writing with a StreamWriter.
For example, set AutoFlush to true when you are writing to a device where the user expects immediate feedback. Console.Out is one of these cases: The StreamWriter used internally for writing to Console flushes all its internal state except the encoder state after every call to StreamWriter.Write.
For a list of common I/O tasks, see Common I/O Tasks.