how can you easily check if access is denied for a file in .NET?

Question

Basically, I would like to check if I have rights to open the file before I actually try to open it; I do not want to use a try/catch for this check unless I have to. Is there a file access property I can check before hand?

Answer 1

Eric Lippert probably explains why Joel is right here

Answer 2

While I agree that this does create a race condition, I believe FileIOPermission has the answer if you really wanted to do it:

new FileIOPermission(FileIOPermissionAccess.Read, path).Demand();

Edit: As pointed out, this is for the FX permissions - not the underlying file system permissions - so it only answers half (the easy half) of your question.

If you're interested in getting NTFS permissions, you'll have to P/Invoke AccessCheck, AuthzAccessCheck, or GetEffectiveRightsFromACL. You'd also need to retrieve the actual ACL from the file, and be wary of enumerating permissions on remote servers.

Answer 3

First, what Joel Coehoorn said.

Also: you should examine the assumptions that underly your desire to avoid using try/catch unless you have to. The typical reason for avoiding logic that depends on exceptions (creating Exception objects performs poorly) probably isn't relevant to code that's opening a file.

I suppose that if you're writing a method that populates a List<FileStream> by opening every file in a directory subtree and you expected large numbers of them to be inaccessible you might want to check file permissions before trying to open a file so that you didn't get too many exceptions. But you'd still handle the exception. Also, there's probably something terribly wrong with your program's design if you're writing a method that does this.

Answer 4

I'll go with PolyglotProgrammer.

Thanks for all the answers.

Answer 5

Quick tip for anyone else coming here with a similar problem:

Watch out for web synchronization apps such as DropBox. I just spent 2 hours thinking the "using" statement (Dispose pattern) is broken in .NET.

I eventually realised that Dropbox is continually reading and writing files in the background, in order to sync them.

Guess where my Visual Studio Projects folder is located? Inside the "My Dropbox" folder of course.

Therefore as I ran my application in Debug mode, the files it was reading and writing were also continually being accessed by DropBox to be synched with the DropBox server. This caused the locking/access conflicts.

So at least I now know that I need to a more robust File Open function (ie TryOpen() that will make multiple attempts). I am surprised it's not already a built-in part of the framework.

[Update]

Here's my helper function:

/// <summary>
/// Tries to open a file, with a user defined number of attempt and Sleep delay between attempts.
/// </summary>
/// <param name="filePath">The full file path to be opened</param>
/// <param name="fileMode">Required file mode enum value(see MSDN documentation)</param>
/// <param name="fileAccess">Required file access enum value(see MSDN documentation)</param>
/// <param name="fileShare">Required file share enum value(see MSDN documentation)</param>
/// <param name="maximumAttempts">The total number of attempts to make (multiply by attemptWaitMS for the maximum time the function with Try opening the file)</param>
/// <param name="attemptWaitMS">The delay in Milliseconds between each attempt.</param>
/// <returns>A valid FileStream object for the opened file, or null if the File could not be opened after the required attempts</returns>
public FileStream TryOpen(string filePath, FileMode fileMode, FileAccess fileAccess,FileShare fileShare,int maximumAttempts,int attemptWaitMS)
{
    FileStream fs = null;
    int attempts = 0;

    // Loop allow multiple attempts
    while (true)
    {
        try
        {
            fs = File.Open(filePath, fileMode, fileAccess, fileShare);

            //If we get here, the File.Open succeeded, so break out of the loop and return the FileStream
            break;
        }
        catch (IOException ioEx)
        {
            // IOExcception is thrown if the file is in use by another process.

            // Check the numbere of attempts to ensure no infinite loop
            attempts++;
            if (attempts > maximumAttempts)
            {
                // Too many attempts,cannot Open File, break and return null 
                fs = null;
                break;
            }
            else
            {
                // Sleep before making another attempt
                Thread.Sleep(attemptWaitMS);

            }

        }

    }
    // Reutn the filestream, may be valid or null
    return fs;
}

Answer 6

public static FileStream GetFileStream(String filePath, FileMode fileMode, FileAccess fileAccess, FileShare fileShare, ref int attempts, int attemptWaitInMilliseconds)
{            
    try
    {
         return File.Open(filePath, fileMode, fileAccess, fileShare);
    }
    catch (UnauthorizedAccessException unauthorizedAccessException)
    {
        if (attempts <= 0)
        {
            throw unauthorizedAccessException;
        }
        else
        {
            Thread.Sleep(attemptWaitInMilliseconds);
            attempts--;
            return GetFileStream(filePath, fileMode, fileAccess, fileShare, ref attempts, attemptWaitInMilliseconds);
        }
    }
}

Answer 7

A simple way to test individual access rights for a given file and user