Simultaneously write to a file from multiple processes

Question

                using (var stream = new FileStream(
                    this.filePath,
                    FileMode.Append,
                    FileAccess.Write,
                    FileShare.ReadWrite))
                {
                    using (var writer = new StreamWriter(stream))
                    {
                        writer.Write("A randomly generated string from process X thread Y");
                        writer.Flush();
                        stream.Flush();
                    }
                }

I am basically using this piece of code in 2 test methods in 2 different Visual Studios (different solutions). I'm running 2 threads per test for 1000 calls to this.

However, when I execute this, all the lines from whichever test I executed first get written to the file, and THEN all the lines get written to the file by the other process. I thought that the writer.Flush() and stream.Flush() calls would prevent this from happening.

How can I write to a file simultaneously from 2 different processes? If I write to a file simultaneously from 2 different threads within the SAME process, the writing occurs as expected (lots of alternating between threads). But for 2 different processes, whichever one is started first does all of its writing, and then the other one goes. Why is this?

Simultaneously write to a file from multiple processes

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