File.WriteAllText and Concurrent Accesses

Question

Suppose I\'m writing a very long string to a file using File.WriteAllText, and another thread or process is trying to read the same file. Would it throw any exception? In ot

Answer 1

MSDN doesn't document which sharing mode is used.

You could look at the source code (either published or via a disassembler), or look at the option at runtime (eg. using Process Monitor and then translating the WIn32 API flags into FileShare value).

But as it isn't documented a patch or new version could change it.

Alternatively if it matters open the file yourself using one of the FileStream overloads that takes a FileShare parameter, open a StreamWriter over this and then write the text.

Would it throw any exception?

Yes. If the file is open already with an incompatible sharing mode the open will fail.

Answer 2

This is the source code from .net Framework 4.0. clearly StreamWriter is used that Uses FileShare.Read Internally.

    [SecuritySafeCritical]
public static void WriteAllText(string path, string contents)
{
    if (path == null)
    {
        throw new ArgumentNullException("path");
    }
    if (path.Length == 0)
    {
        throw new ArgumentException(Environment.GetResourceString("Argument_EmptyPath"));
    }
    InternalWriteAllText(path, contents, StreamWriter.UTF8NoBOM);
}


private static void InternalWriteAllText(string path, string contents, Encoding encoding)
{
    using (StreamWriter writer = new StreamWriter(path, false, encoding))
    {
        writer.Write(contents);
    }
}

This is the code that creates the underlying stream for StreamWriter.

private static Stream CreateFile(string path, bool append)
{
    return new FileStream(path, append ? FileMode.Append : FileMode.Create, FileAccess.Write, FileShare.Read, 0x1000, FileOptions.SequentialScan);
}

Answer 3

Would it throw any exception?

Yes. You should ensure that while one process is writing to a file others are not reading to it by using a lock. Even if you set the FileShare parameter to Read for example which would allow subsequent openings of the file for reading and not throw an exception immediately it is not a good idea as those readers would probably get corrupt results.