Add "Everyone" privilege to folder using C#.NET

I have used the code below to allow Everyone access to a folder:

System.Security.AccessControl.DirectorySecurity sec =
    System.IO.Directory.GetAccessControl(directory, AccessControlSections.All);
FileSystemAccessRule accRule = new FileSystemAccessRule("Everyone",
                                       FileSystemRights.Modify,
                                       AccessControlType.Allow);
sec.AddAccessRule(accRule);    // setACL
sec.ResetAccessRule(accRule);

Now, the Everyone user is added to the folder, but not with any rights assigned. All the read, write, execute etc. checkboxes are not checked.