I try to enable windows account with this code but i ve exception in ObjectSecurity (Null reference) in this line ActiveDirectorySecurity userSecurity = user.ObjectSecurity;
I would suspect that your error would be on the 5th line in the code
user.Properties["userAccountControl"].Value = val & ~0x2; //this will error
and if you were to debug it you would probably see the following error
Error Pointers and fixed size buffers may only be used in an unsafe context