FileStream fs1 = new FileStream("TestFile", FileMode.Open, FileAccess.Write, FileShare.Read);
FileStream fs2 = new FileStream("TestFile", FileMode.Open, FileAccess.Read, FileShare.Read);
IOException will be thrown about "The process cannot access the file ... because it is being used by another process."
Resolution:
Set FileShare parameter to Write or ReadWrite for fs2.
The FileShare param. specifies whether you want the file to be shared for Read, Write or Both, if you only specify Read, you can't open the file because fs1 has it open for Write.