set file permissions for c:\program files\company\app\file for all users
FileSecurity access = file.GetAccessControl();
SecurityIdentifier everyone = new SecurityIdentifier(WellKnownSidType.WorldSid, null);
access.AddAccessRule(new FileSystemAccessRule(everyone, FileSystemRights.ReadAndExecute, AccessControlType.Allow));

file.SetAccessControl(access);

public class PermissionController
{
    private readonly string _file;
    private readonly FileSecurity _accessControl;
    private readonly SecurityIdentifier _id;
    private readonly List<FileSystemAccessRule> _permissionsDenied;

    public PermissionController(string file)
    {
        _file = file;
        _accessControl = File.GetAccessControl(_file);
        _id = WindowsIdentity.GetCurrent().Owner;
        _permissionsDenied = new List<FileSystemAccessRule>();
    }

    public void Allow(params FileSystemRights[] rights)
    {
        foreach (var right in rights)
            AddRule(Rule(right, AccessControlType.Allow));
    }

    public void Deny(params FileSystemRights[] rights)
    {
        foreach (var right in rights)
        {
            var rule = Rule(right, AccessControlType.Deny);
            AddRule(rule);
            _permissionsDenied.Add(rule);
        }
    }

    private void AddRule(FileSystemAccessRule rule)
    {
        _accessControl.AddAccessRule(rule);
    }

    private FileSystemAccessRule Rule(FileSystemRights right, AccessControlType type)
    {
        return new FileSystemAccessRule(_id, right, type);
    }

    public void RemoveDeniedPermissions()
    {
        foreach (var rule in _permissionsDenied)
            _accessControl.RemoveAccessRule(rule);

        Apply();
    }

    public void Apply()
    {
        File.SetAccessControl(_file,_accessControl);
    }
}

_permissionController = new PermissionController(_file);
        _permissionController.Allow(FileSystemRights.Read, FileSystemRights.Write);
        _permissionController.Deny(FileSystemRights.FullControl,
                                   FileSystemRights.Modify,
                                   FileSystemRights.ReadAndExecute);
        _permissionController.Apply();