C#などで使う、System.IO.FileStream.
こいつのオプションの組み合わせでいつも頭を抱えていたのでメモ

FileMode vs FileAccess

o : Success
x : ArgumentException
IO : IOException

File doesn’t exist

FileMode \ FileAccess	Read	Write	ReadWrite
CreateNew	x	o	o
Create	x	o	o
Open	IO	IO	IO
OpenOrCreate	o	o	o
Truncate	x	IO	IO
Append	x	o	x

File already exist

FileMode \ FileAccess	Read	Write	ReadWrite
CreateNew	x	IO	IO
Create	x	o	o
Open	o	o	o
OpenOrCreate	o	o	o
Truncate	x	o	o
Append	x	o	o

If file is already opened by FileMode

Already opened by FileMode	Needs to specify to open FileShare
Read	Read or Write
Write	Write or ReadWrite
ReadWrite	Read or Write or ReadWrite

If file is already opended by FileShare

Already opened by FileShare	Needs to specify to open FileMode
None	Cannot open
Read	Read
Write	Write
ReadWrite	Read or Write or ReadWrite