C# Edit a variable of arraylist index

Question

I got an arraylist of rectangles and I want to edit the X & Y values of all of them.

I tried using a foreach loop

        foreach (Rectangle rect in rectangles)
            rect.X += 1;

But this wouldn't work as it's read-only, so I tried a regular for loop

        for (int i = 0; i < rectangles.Count; i++)
            rectangles[i].X += 1;

And for some reason this wouldn't work either, because rectangles[i] just doesn't have any of rectangle's methods.

Then I stumbled across a post somewhere on stackoverflow about how to call methods of elements of an arraylist. And I haven't been able to find examples of this, so I hope someone can clear this up.

(Unit.unitArray[selectedUnit] as MyClass).DisplayUnitAttributes()

But I have no idea how to put this to use, I don't understand what Unit is supposed to be replaced with, and I'm guessing MyClass would be Rectangle...

Any help is appreciated!

TLDR: I want to iterate over an arraylist with rectangles, and edit the X & Y values of them.

Could you sum up the differences for me? I'm used to Java where ArrayList is an interface of List. — SJ19
– SJ19, Commented Oct 4, 2015 at 6:48
@GrantWinney Tried for (int i = 0; i < rectangles2.Count; i++) rectangles2[i].X += 1; where rectangles2 is a List<Rectangle>, doesn't work because "Return value is not a variable"... — SJ19
– SJ19, Commented Oct 4, 2015 at 6:54
@M.kazemAkhgary That doesn't work, and I don't know what a struct is. — SJ19
– SJ19, Commented Oct 4, 2015 at 7:02
@M.kazemAkhgary Btw, I'm using C#'s Rectangle class, I just want a way to edit values of it in an arraylist, I'm getting linked everywhere, it's not helping me. Can you please just answer my question. — SJ19
– SJ19, Commented Oct 4, 2015 at 7:11
I like how everyone is deleting their answers and comments, please only post if you actually know what you're doing. — SJ19
– SJ19, Commented Oct 4, 2015 at 7:14

M.kazem Akhgary · Accepted Answer · 2015-10-04 07:26:04Z

1

According to the error you have got. "Return value is not a variable"

Rectangle is not a class. its an struct which is not reference type so you have to assign new value into it.

List<Rectangle> rectangles = new List<Rectangle>();

// rectangles.Add(x); make your list here

for (int index = 0; index < rectangles.Count; index++)
{
    Rectangle r = rectangles[index];
    r.x += 1;
    rectangles[index] = r;
}

edited Oct 4, 2015 at 7:26

answered Oct 4, 2015 at 7:17

M.kazem Akhgary

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Community · Accepted Answer · 2017-05-23 11:51:52Z

0

It's act like this because ArrayList is not typed. It's better to use a List<T> instead (where T is your class, in your example it will be a Rectangle). Here is MSDN link.

You can't modify object this way in the foreach loop. More info on this SO question.

About this example: (Unit.unitArray[selectedUnit] as MyClass).DisplayUnitAttributes() It's called casting. You can read more about this on this SO question. So in your code it should looks like this: (rectangles[i] as Rectangle).X += 1;. You also should check if result of this casting is not null etc. But better read previous link to understand this.

edited May 23, 2017 at 11:51

CommunityBot

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answered Oct 4, 2015 at 7:09

Grzegorz Piotrowski

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2 Comments

SJ19 Over a year ago

I know what casting is, and I tried ---- for (int i = 0; i < rectangles2.Count; i++) (rectangles[i] as Rectangle).X += 1; ---- but it doesn't work.

Grzegorz Piotrowski Over a year ago

Ah, okey, your Rectangle is a struct, not a class. Sorry! Second answer is good then ;)

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