advice on TLink please

For a space invaders clone, you'll be aware only one invader moves per frame. I'm having a problem whereby in my mainloop I check all invaders and remove those in collision from the alienObjectList

In addition, in order to move one invader per frame, I set a variable before my main loop:

Link:TLink = AlienObjectList.FirstLink()

Then inside the main loop I do this:

for each
alien.draw() ' draw all aliens
alien.collisionCheck() ' remove from AlienObjectList if collision
next

finally I use the Link variable to update the position of one invader per frame...

If Link <> Null
Local MyObject:TAlien1 = TAlien1(Link.Value()) 'cast object
MyObject.updatestate() ' update the onscreen position
Link = Link.NextLink()

Else
Link = AlienObjectList.FirstLink()
EndIf

I'm getting crashes during play because I think when doing checkCollision() and removing an alien from the list, its messing up the Link variable. Is there a better way ?

Error: Attempt to access field or method of Null Object

Don't use a link to keep track of which alien you are at, but use an index variable instead. Something like

Local i:Int = 0
...
'Mainloop:

i :+ 1
If i>list.Count() Then i = 0
Local j:Int

For alien = EachIn list
alien.draw() ' draw all aliens
alien.collisionCheck() ' remove from AlienObjectList if collision
If i=j Then alien.UpdateState()
j :+ 1
Next

As a personal opinion I wouldnt bother storing the invaders in a list. I went down that same route when I wrote mine and eventually I found it far easier to treat the invaders as a single InvaderWave object. It would store the individual invaders in a 2d array and keep track of what direction they should be moving, what individual should be moving next, and which invader should be dropping a bomb.

I used TLists for storing the falling bombs and the explosions which I upgraded to a simple particle effect explosion.

OTUS
thanks - will give it a try
EDITED: Otus, I implemented your change and it's working perfectly. Thanks

QuietBloke
Yes, I thought about an array which would certainly be much easier as there's a known quantity of invaders and each could be given a display flag based on whether they've been hit or not. But this was meant to be an exercise for me to get my head around types and lists and OO, so I ignored that route. Might get a separate version running like this.

thats fine.. no better way to learn than to get stuck in. Good luck. :)