In Business layer under (3 tier) how implement login form authentication

public void btnLogin_click()
{
    //get both username & password and store it in string variables

    LoginControlClass logClass = new LoginControlClass();
logClass.IsAuthenticatedUser(userName, password);
}

public class LoginControlClass
{

   public bool IsAuthenticatedUser(string userName, string password)
   {
     //put in your logic to authenticate.
//If LDAP, get details from LDAP to validate uname/password
//If custom, then invoke data from DAL to validate username/ password.
LoginDal loginDal = new LoginDal();
User user = loginDal.RetrieveUserCredentials(userName, password);
//Check the variable inside user object to determine the authentication status.

 }


}

public class LoginDal
{
  public User RetrieveUserCredentials()
  {
//Query from DB to prepare User object. This can be a DTO. Remember not to save the password in User object, instead you may think of set the validated authentication result in a variable.
}

}