C# Test if user has write access to a folder

Question

I appreciate that this is a little late in the day for this post, but you might find this bit of code useful.

string path = @"c:\temp";
string NtAccountName = @"MyDomain\MyUserOrGroup";

DirectoryInfo di = new DirectoryInfo(path);
DirectorySecurity acl = di.GetAccessControl(AccessControlSections.All);
AuthorizationRuleCollection rules = acl.GetAccessRules(true, true, typeof(NTAccount));

//Go through the rules returned from the DirectorySecurity
foreach (AuthorizationRule rule in rules)
{
    //If we find one that matches the identity we are looking for
    if (rule.IdentityReference.Value.Equals(NtAccountName,StringComparison.CurrentCultureIgnoreCase))
    {
        var filesystemAccessRule = (FileSystemAccessRule)rule;

        //Cast to a FileSystemAccessRule to check for access rights
        if ((filesystemAccessRule.FileSystemRights & FileSystemRights.WriteData)>0 && filesystemAccessRule.AccessControlType != AccessControlType.Deny)
        {
            Console.WriteLine(string.Format("{0} has write access to {1}", NtAccountName, path));
        }
        else
        {
            Console.WriteLine(string.Format("{0} does not have write access to {1}", NtAccountName, path));
        }
    }
}

Console.ReadLine();

Drop that into a Console app and see if it does what you need.

public bool IsDirectoryWritable(string dirPath, bool throwIfFails = false)
{
    try
    {
        using (FileStream fs = File.Create(
            Path.Combine(
                dirPath, 
                Path.GetRandomFileName()
            ), 
            1,
            FileOptions.DeleteOnClose)
        )
        { }
        return true;
    }
    catch
    {
        if (throwIfFails)
            throw;
        else
            return false;
    }
}

That's a perfectly valid way to check for folder access in C#. The only place it might fall down is if you need to call this in a tight loop where the overhead of an exception may be an issue.

There have been other similar questions asked previously.

I tried most of these, but they give false positives, all for the same reason.. It is not enough to test the directory for an available permission, you have to check that the logged in user is a member of a group that has that permission. To do this you get the users identity, and check if it is a member of a group that contains the FileSystemAccessRule IdentityReference. I have tested this, works flawlessly..

    /// <summary>
    /// Test a directory for create file access permissions
    /// </summary>
    /// <param name="DirectoryPath">Full path to directory </param>
    /// <param name="AccessRight">File System right tested</param>
    /// <returns>State [bool]</returns>
    public static bool DirectoryHasPermission(string DirectoryPath, FileSystemRights AccessRight)
    {
        if (string.IsNullOrEmpty(DirectoryPath)) return false;

        try
        {
            AuthorizationRuleCollection rules = Directory.GetAccessControl(DirectoryPath).GetAccessRules(true, true, typeof(System.Security.Principal.SecurityIdentifier));
            WindowsIdentity identity = WindowsIdentity.GetCurrent();

            foreach (FileSystemAccessRule rule in rules)
            {
                if (identity.Groups.Contains(rule.IdentityReference))
                {
                    if ((AccessRight & rule.FileSystemRights) == AccessRight)
                    {
                        if (rule.AccessControlType == AccessControlType.Allow)
                            return true;
                    }
                }
            }
        }
        catch { }
        return false;
    }

IMHO the only 100% reliable way to test if you can write to a directory is to actually write to it and eventually catch exceptions.