File.Open Cross Thread Writing

Question

Is it safe to open a file and write a stream to it across multiple threads? If no what could I implement to make it safe?

Answer 1

No, the standard FileStream is not thread-safe.

You should use var safe = Stream.Synchronized(File.Open(...)); to create a thread-safe wrapper.

Stream.Synchronized() on MSDN

Answer 2

No; it isn't safe by itself, but you can use the lock keyword to control cross-thread access to the FileStream object.

For instance:

FileStream fStream = File.Open('foo.txt', FileMode.Append);

lock (fStream)
{
  // Some operation on fStream
}

The answer posted by Knaģis may be the way to go in this instance, but mutex locking with the lock keyword is a more general solution.

Answer 3

No. This is not thread safe. Intuitively, how does one write to a file when the index is a moving target? One idea is to create separate files and combine them with another process, possibly triggered by a completed event of some kind (e.g. all files arrived).

The concept of writing to a single file with multiple threads has some code smell. IMO, re-examine your options and come up with a different way of approaching this problem.