FileSecurity fSecurity = File.GetAccessControl(filepath);


//[color=red]exception:[/color] some or all identity references could not be translated
fSecurity.AddAccessRule(new FileSystemAccessRule("Everyone", FileSystemRights.FullControl, AccessControlType.Allow));

//[color=red]exception:[/color] some or all identity references could not be translated
fSecurity.AddAccessRule(new FileSystemAccessRule(System.Environment.MachineName + @"\IIS_IUSRS", FileSystemRights.FullControl, AccessControlType.Allow));

//[color=red]exception:[/color] запрещено задание флагов
SecurityIdentifier everyone = new SecurityIdentifier(WellKnownSidType.WorldSid, null);
fSecurity.AddAccessRule(new FileSystemAccessRule(everyone, FileSystemRights.Modify | FileSystemRights.Synchronize, InheritanceFlags.ContainerInherit | InheritanceFlags.ObjectInherit, PropagationFlags.None, AccessControlType.Allow));

FileIOPermission fp = new FileIOPermission(FileIOPermissionAccess.Read, Folderpath);
                fp.AddPathList(FileIOPermissionAccess.AllAccess, Filepath);
                try
                {
                    fp.Demand();
                }
                catch (SecurityException s)
                {

                }

FileSecurity fSecurity = File.GetAccessControl(filepath);

SecurityIdentifier everyone = new SecurityIdentifier(WellKnownSidType.WorldSid, null);
fSecurity.AddAccessRule(new FileSystemAccessRule(everyone, FileSystemRights.FullControl, AccessControlType.Allow));

File.SetAccessControl(filepath, fSecurity);