IOException when trying to read a file using File.ReadAllBytes function

You might get an exception that says “The process cannot access the file C:\bla\bla.bin because it is being used by another process” when trying to read all bytes at once using File.ReadAllBytes(). While it is arguable if you should be reading an entire file into memory at once, but in here we’ll assume you do have a good reason. Anyways, so why does the error occur? That’s what I want to talk about.

File.ReadAllBytes() function is written such that it uses a call like the following to instantiate a FileStream:

FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read)

How do I know this? Well, there’s a mass-downloader tool for downloading .NET 2.0 code that Microsoft made available a couple of months ago. Before that there was Reflector.

Notice that the FileShare is specified as Read. That means if you have another piece of code writing to the same file at the same time, you will get an error even if the other piece of code created the FileStream by specifying the share mode as “ReadWrite” like so:

FileStream fs = new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.ReadWrite)

So what’s the solution? Well we need to open the FileStream while reading with a share mode of “ReadWrite” instead of “Read”. You cannot change what ReadAllBytes does. So if you need functionality like that, you are going to have to implement a similar function on your own.

Here’s how you can do that:

public static byte[] ReadAllBytes(String path) { byte[] bytes; using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)) { int index = 0; long fileLength = fs.Length; if (fileLength > Int32.MaxValue) throw new IOException("File too long"); int count = (int)fileLength; bytes = new byte[count]; while (count > 0) { int n = fs.Read(bytes, index, count); if (n == 0) throw new InvalidOperationException("End of file reached before expected"); index += n; count -= n; } } return bytes; }