Putting It All Together (2)

Now, we want to loop through all the remainingInvaders. Then tower could firing on the invaders.

public bool Play()
{
    // run until all invaders are neutralized or reach the end of map
    int remainingInvaders = _invaders.Length;

    while (remainingInvaders>0)
    {
        // each tower has opportunity to fire on invaders    
        foreach(Tower tower in towers)
        {
            tower.FireOnInvaders(_invaders);
        }

        // count and move the invaders that are still active

    }
}

After that, we would like to count how much invaders still alive.

public bool Play()
{
    // run until all invaders are neutralized or reach the end of map
    int remainingInvaders = _invaders.Length;

    while (remainingInvaders>0)
    {
        // each tower has opportunity to fire on invaders    
        foreach(Tower tower in towers)
        {
            tower.FireOnInvaders(_invaders);
        }

        // count and move the invaders that are still active
        remainingInvaders = 0;
        foreach(Invader invader in _invaders)
        {

        }
    }
}

In each remaing and active invader, we want them to move and check if they have reach the end of map.

public bool Play()
{
    // run until all invaders are neutralized or reach the end of map
    int remainingInvaders = _invaders.Length;

    while (remainingInvaders>0)
    {
        // each tower has opportunity to fire on invaders    
        foreach(Tower tower in towers)
        {
            tower.FireOnInvaders(_invaders);
        }

        // count and move the invaders that are still active
        remainingInvaders = 0;
        foreach(Invader invader in _invaders)
        {
            if (invader.IsActive)
            {
                invader.Move();

                if(invader.HasScored)
                {
                    return false;
                }

                remainingInvaders++;
            }
        }
    }

    return true;
}