File.Open vs new FileStream()

Question

I know File.Open() internally calls new FileStream(). So why does code utilizing File.Open() throw an IOException with the error message:

The process cannot access the file because it is being used by another process

Code with using FileStream() does not cause this error.

for (int i = 0; i < 20000; i++)
{
    XmlSerializer x1 = new XmlSerializer(typeof(Tasks));
    using (FileStream fileStream = File.Open(
             Settings.Default.DownloadJobFileName,
             FileMode.Open, 
             FileAccess.Read))
        {
            tasks = ((Tasks)xmlSerializer.Deserialize(fileStream));
        }

    XmlSerializer x2 = new XmlSerializer(typeof(Tasks));
    using (FileStream fileStream = File.Open(
              Settings.Default.DownloadJobFileName,
              FileMode.Create, 
              FileAccess.Write))
        {
            xmlSerializer.Serialize(fileStream, tasks);
        }
}

I'm using Windows 8.1 and .Net 4.0 client profile, seems it's getting worse on Windows Embedded Standard 2009

Answer 1

The FileShare options of the constructors/methods you use are different:

public FileStream(String path, FileMode mode, FileAccess access) 
    : this(path, mode, access, FileShare.Read,
           DefaultBufferSize, FileOptions.None, Path.GetFileName(path), false) {
}

compared to

public static FileStream Open(String path, FileMode mode, FileAccess access) {
    return Open(path,mode, access, FileShare.None);
}

public static FileStream Open(String path, FileMode mode, FileAccess access, FileShare share) {
    return new FileStream(path, mode, access, share);
}

When you create a new FileStream other processes can still read the file. File.Open on the other hand locks the file so no other process can access it.