GetEffectiveRightsFromAcl throws invalid acl error

I am trying to get the effective rights a user has on a file using interop in C#. Following i开发者_Go百科s the code I am using :

        public static FileSystemRights GetFileEffectiveRights(string FileName, string UserName)
    {
        IntPtr pDacl, pZero = IntPtr.Zero;
        int Mask = 0;
        uint errorReturn = GetNamedSecurityInfo(FileName, SE_OBJECT_TYPE.SE_FILE_OBJECT, SECURITY_INFORMATION.Dacl
            , out pZero, out pZero, out pDacl, out pZero, out pZero);
        if (errorReturn != 0)
        {
            throw new Exception("Win error : " + errorReturn);
        }
        Program.TRUSTEE pTrustee = new TRUSTEE();
        pTrustee.pMultipleTrustee = IntPtr.Zero;
        pTrustee.MultipleTrusteeOperation = (int)Program.MULTIPLE_TRUSTEE_OPERATION.NO_MULTIPLE_TRUSTEE;
        pTrustee.ptstrName = UserName;
        pTrustee.TrusteeForm = (int)Program.TRUSTEE_FORM.TRUSTEE_IS_NAME;
        pTrustee.TrusteeType = (int)Program.TRUSTEE_TYPE.TRUSTEE_IS_USER;
        errorReturn = GetEffectiveRightsFromAcl(pDacl, ref pTrustee, ref Mask);
        if (errorReturn != 0)
        {
            throw new Exception("Win error : " + errorReturn);
        }
        return (FileSystemRights)Mask;
    }

This code works fine until I start modifying the ACL structure using the classes FileAccessRule and FileInfo, and then I start getting Windows Error 1336 : ERROR_INVALID_ACL. Same is the case if I debug the process : I call GetFileEffectiveRights once, pause the process,change the ACL through windows API, and resume and call GetFileEffectiveRights again(the 1st call succeeds but the second gives 1336.)

What is going wrong?

PS : I am developing on Windows 7 using VS 2008 and .NET 3.5

EDIT : I only get the error when I try to get rights for a file for which a non-inherited ACE was added through the Windows GUI/ C#'s File API.

The problem is in the other part of your problem which change the ACL through windows API.

The order of ACEs in DACL is very important. In http://support.microsoft.com/kb/269175/en for example you will find the full description about correct ACE order and a code example.

If you will stay have the same problem, just post the code example of modification of ACL in your question.