How to lock a file with C#?

Question

I'm not sure what people usually mean by "lock" a file, but what I want is to do that thing to a file that will produce a "The specified file is in use" error message when I try to open it with another application.

I want to do this to test my application to see how it behaves when I try to open a file that is on this state. I tried this:

FileStream fs = null;

private void lockToolStripMenuItem_Click(object sender, EventArgs e)
{
    fs = new FileStream(@"C:\Users\Juan Luis\Desktop\corte.txt", FileMode.Open);
}

private void unlockToolStripMenuItem_Click(object sender, EventArgs e)
{
    fs.Close();
}

But apparently it didn't do what I expected because I was able to open the file with Notepad while it was "locked". So how can I lock a file so it cannot be opened with another application for my testing purposes?

Kissinger

Answer 1

As per http://msdn.microsoft.com/en-us/library/system.io.fileshare(v=vs.71).aspx

FileStream s2 = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.None);

Cabby

Note : opening a FileStream on a file works perfectly as a lock in Windows, but does not appear to have any effect in Linux. – Uda

Answer 2

You need to pass in a FileShare enumeration value of None to open on the FileStream constructor overloads:

fs = new FileStream(@"C:\Users\Juan Luis\Desktop\corte.txt", FileMode.Open, 
    FileAccess.ReadWrite, FileShare.None);

Polychromy

Wish I can sometimes accept both answers as they are pretty much exactly the same. Hope you don't mind I'm accepting the other guy because of his lower score :) – Kissinger

Answer 3

While FileShare.None is undoubtedly a quick and easy solution for locking a whole file you could lock part of a file using FileStream.Lock()

public virtual void Lock(
    long position,
    long length
)

Parameters

position
    Type: System.Int64
    The beginning of the range to lock. The value of this parameter must be equal to or greater than zero (0). 

length
    Type: System.Int64
    The range to be locked. 

and conversely you could use the following to unlock a file: FileStream.Unlock()

public virtual void Unlock(
    long position,
    long length
)

Parameters

position
    Type: System.Int64
    The beginning of the range to unlock. 

length
    Type: System.Int64
    The range to be unlocked. 

Asquint

Answer 4

There are multiple ways to lock a file i.e. disallow file sharing. Each makes sense in a specific scenario.

1. Use the static File.Open (or any other opening method of the static File API) and pass in FileShare.None.
   It's important to understand that once the using scope is left i.e. the FileStream was disposed the file lock is released:

   await using FileStream lockedFile = File.Open(
     "test.txt", 
     FileMode.OpenOrCreate, 
     FileAccess.ReadWrite, 
     FileShare.None);
   

2. Use the FileStream constructor and pass in FileShare.None.
   It's important to understand that once the using scope is left i.e. the FileStream was disposed the file lock is released:

   await using FileStream lockedFile = File.Open(
     "test.txt", 
     FileMode.OpenOrCreate, 
     FileAccess.ReadWrite, 
     FileShare.None);
   

3. If you need a more control over the file lock, use FileStream.Lock to acquire a file lock. FileStream.Lock requires a position in the stream and the length as an argument to specify the locked content. This adds more flexibility as we can lock the complete file or only a part or parts of the file (partial file lock).

   Generally, use FileStream.Unlock to explicitly release the lock from the file or a part of the file. This member is useful to implement a temporary lock without having to dispose the current FileStream.

   await using var fileStream = new FileStream("test.txt", FileMode.Open);
   
   // Lock the first 1024 Bytes of the file
   fileStream.Lock(0, 1024);
   
   // Lock the last 1024 Bytes of the file
   fileStream.Lock(fileStream.Length - 1024, 1024);
   
   // Release the lock for the first 1024 Bytes 
   // but keep the lock for the last 1024 Bytes 
   fileStream.Unlock(0, 1024);
   
   // Lock the complete file
   fileStream.Lock(0, fileStream.Length);
   
   // Unlock the complete file
   fileStream.Unlock(0, fileStream.Length);
   

Phlogopite

Answer 5

I've needed this frequently enough to add this to my $PROFILE to use from PowerShell:

function Lock-File {
    Param( 
        [Parameter(Mandatory)]
        [string]$FileName
    )

    # Open the file in read only mode, without sharing (I.e., locked as requested)
    $file = [System.IO.File]::Open($FileName, 'Open', 'Read', 'None')

    # Wait in the above (file locked) state until the user presses a key
    Read-Host "Press Return to continue"

    # Close the file (This releases the current handle and unlocks the file)
    $file.Close()
}

Bituminous