WPF list modification during iteration? - wpf

Question

I'm trying to make a very simple game where the yellow ball bouncing back and fourth. If it collides with one of the moving blue squares, the square is supposed to disappear and a new one should appear (always 3 in the window) elsewhere. When my code reaches this part, all 3 squares disappear (then reappears as intended it is not a problem) and I just cant figure out why. It would be a huge help if somebody could run over my methods responsible for the problem. Thank you in advance.
So my timer_Tick method, responsible for every frame:
void timer_Tick(object sender, EventArgs e)
{
logic.MoveBall();
if (model.Enemy.Count<3)
{
logic.AddEnemy();
}
int iii = 0;
foreach (MyShape enemy in model.Enemy) //the whole thing from here is me trying to solve list modification during iteration
{
if (logic.MoveEnemy(enemy) == -1)
{
logic.MoveEnemy(enemy);
}
else iii = logic.MoveEnemy(enemy);
}
if (iii > -1)
{
for (int j = model.Enemy.Count - 1; j >= 0; j--)
{
if (j == model.Enemy.Count - iii)
{
model.Enemy.RemoveAt(j);
}
}
}
}
MoveEnemy: I try to decide whether there is collusion and if yes, then try to remove the given shape object (blue square). Because This whole method is in a foreach, I just save the removable element and forward it to timer_Tick
public int MoveEnemy(MyShape shape)
{
int i = 0;
int ii = -1;
if ((shape.Area.IntersectsWith(model.Ball.Area)))
{
i = 0;
foreach (var e in model.Enemy)
{
i++;
if (shape == e)
{
ii = i;
}
}
}
shape.ChangeX(shape.Dx);
shape.ChangeY(shape.Dy);
bool coll = false;
foreach (var e in model.Enemy)
{
if ((e.Area.IntersectsWith(shape.Area)) && (shape != e))
{
coll = true;
}
}
if (shape.Area.Left < 0 || shape.Area.Right > Config.Width-40 || coll)
{
shape.Dx = -shape.Dx;
}
if (shape.Area.Top < 0)
{
shape.Dy = -shape.Dy;
}
if (shape.Area.Bottom > Config.Height/2)
{
shape.Dy = -shape.Dy;
}
RefreshScreen?.Invoke(this, EventArgs.Empty);
return ii;
}
And finally AddEnemy:
public void AddEnemy()
{
rnd = new Random();
int r = rnd.Next(-300, 300);
model.Enemy.Add(new MyShape(Config.Width / 2+r, 0, 40, 40));
RefreshScreen?.Invoke(this, EventArgs.Empty);
}

Answer 1

List<T> (or IList and Enumerable) exposes some useful methods to compact code:
int itemIndex = list.IndexOf(item); // Gets the index of the item if found, otherwise returns -1
list.Remove(item); // Remove item if contained in collection
list.RemoveAll(item => item > 5); // Removes all items that satisfy a condition (replaces explicit iteration)
bool hasAnyMatch = list.Any(item => item > 5); // Returns true as soon as the first item satisfies the condition (replaces explicit iteration)
A simplified version, which should eliminate the flaw:
void timer_Tick(object sender, EventArgs e)
{
if (model.Enemy.Count < 3)
{
logic.AddEnemy();
}
logic.MoveBall();
model.Enemy.ForeEach(logic.MoveEnemy);
model.Enemy.RemoveAll(logic.IsCollidingWithBall);
}
public void AddEnemy()
{
rnd = new Random();
int r = rnd.Next(-300, 300);
model.Enemy.Add(new MyShape(Config.Width / 2 + r, 0, 40, 40));
RefreshScreen?.Invoke(this, EventArgs.Empty);
}
public bool IsCollidingWithBall(MyShape shape)
{
return shape.Area.IntersectsWith(model.Ball.Area);
}
public int MoveEnemy(MyShape shape)
{
shape.ChangeX(shape.Dx);
shape.ChangeY(shape.Dy);
bool hasCollision = model.Enemy.Any(enemy => enemy.Area.IntersectsWith(shape.Area)
&& enemy != shape);
if (hasCollision || shape.Area.Left < 0 || shape.Area.Right > Config.Width - 40)
{
shape.Dx = -shape.Dx;
}
if (shape.Area.Top < 0)
{
shape.Dy = -shape.Dy;
}
if (shape.Area.Bottom > Config.Height / 2)
{
shape.Dy = -shape.Dy;
}
RefreshScreen?.Invoke(this, EventArgs.Empty);
}