Question

Can anyone tell me why I am getting this exception? Well I know “why” I am getting it is cause it says that the element in the list I am trying to access does not exist, but stepping through the code I can see that it actually does exist?

In Game1 I call a helper class’ calculation method. Then when the calculation is done, I add the result to a List<> in the CalculationHelper class.
The code for this looks like:

In Game1 class where I have instanciated CalculationHelper calcHelper class and then call one of the class methods.
Fisrt, in Game1 I call calcHelper.ForceVanishingPointIntersections(…) method in a for() loop which can be seen below. This works just fine and the calcHelper.ForceVanishingPointIntersections(…) method adds the value to the IntersectionPointList<> in the CalculationHelper class (see method below).

for (int i = 0; i < LineList.Count; i++)
{
    calcHelper.ForceVanishingPointIntersections(LineList[i], LineList[i + 1]);

    AddSprite(VanishingPointIntersectionList, calcHelper.GetIntersectionPointListElementAt(i), greenCirlce);
    i++;
}

To do some calculations and add a value to the IntersectionPointlist in CalculationHelper class I do:

List<Vector2> IntersectionPointList = new List<Vector2>();
public void ForceVanishingPointIntersections(Line line1, Line line2)
{
    Vector2 intersectionPoint;
    // Do some calculations

    IntersectionPointList.Add(intersectionPoint);
}

Finally in Game1 class, the second method in the for() loop I call AddSprite to create a Sprite. I want to pass in the values stored in CalculationHelper class IntersectionPointList as coordinates for the Sprite.
To this end I call calcHelper.GetIntersectionPointListElementAt(i) which calls a method in CalculationHelper class like so (which should return the value at the specified point (i) from the IntersectionPointList<>)
public Vector2 GetIntersectionPointListElementAt(int index)
{
Vector2 result = this.IntersectionPointList[index];
return result;
}
The first time the for() loop executes, this works fine. I do the calculations, the value is stored in the list and I am able to get this value from the list and pass it to AddSprite(..). However, the second time round the for() loop, when I call GetIntersectionPointListElementAt from the AddSprite() method, I get an exception in my

public Vector2 GetIntersectionPointListElementAt(int index)
{
    Vector2 result = this.IntersectionPointList[index];    // Exception thrown here
    return result;
}

saying the index was out of range? But stepping through the code, my IntersectionPointList is updated and it shows that the list now contains 2 values. And I’m trying to access the second value.

Does anyone have an idea why this could be?
For the life of me I cant figure out where I am going wrong.

If more code is needed I can post some more, just let me know

Answer 1

Because you access LineList[] with an index i + 1 you must diminish the last index by one in the for-condition. (Note the -1)

for (int i = 0; i < LineList.Count - 1; i++) {
    calcHelper.ForceVanishingPointIntersections(LineList[i], LineList[i + 1]);
    AddSprite( ... );
}

This will call ForceVanishingPointIntersections with the indexes

(0, 1), (1, 2), (2, 3), ... (Count-2, Count-1)

Note that the index range of LineList is 0 ... Count-1.

---

If non-overlapping indexes are required instead, like

(0, 1), (2, 3), (4, 5), ... (Count-2, Count-1)

then change the loop to

for (int i = 0; i < LineList.Count - 1; i += 2) {
    calcHelper.ForceVanishingPointIntersections(LineList[i], LineList[i + 1]);
    AddSprite(
        VanishingPointIntersectionList,
        calcHelper.GetIntersectionPointListElementAt(i / 2),
        greenCirlce);
}

EDIT According the Chris Gessler’s comments this second approach is the right one.

Remove the i++ inside the loop, it’s quite uncommon and confusing. Instead, replace it by i += 2 in loop header.

Also note that (again according to Chris Gessler) IntersectionPointList has half as much items as LineList. Therefore call GetIntersectionPointListElementAt with i / 2. Since i is {0, 2, 4, ... }, i / 2 is {0, 1, 2, ...}.

---

The for-loop allows you to have a comma-separated list of statements in the first and third section. You could use it to maintain two indexes

for (int i = 0, k = 0; i < LineList.Count - 1; i += 2, k++) {
    calcHelper.ForceVanishingPointIntersections(LineList[i], LineList[i + 1]);
    AddSprite(
        VanishingPointIntersectionList,
        calcHelper.GetIntersectionPointListElementAt(k),
        greenCirlce);
}