An object reference error messages is required for the non static field or property method

Question

I have one problem, as shown below, although both are public and BUS 2 class has a reference to DAO, but still an object reference error messages is required for the non static field or property method.

Image DAO Image BUS

Answer 1

add static ex: public static SYS_TaiKhoan GetTaiKhoan(string username) { return HeThongDao.GetTaiKhoan(username); }