Cannot modify the return value of a collection of rectangles

Question

Here's what I used to solve this problem:

public class Snake {
List<Rectangle> bodyParts = new List<Rectangle>();
}

Snake snk = new Snake();
snk.bodyParts.Add(new Rectangle(760,25,8,8))

//the Exception Occurs here
//snk.bodyParts[0].Y = snk.bodyPArts[0].Y-10;

//Here's my approach
snk.bodyParts[0] = new Rectangle(bodyParts[0].X,bodyParts[0].Y-10,8,8);

Exception: Cannot modify the return value of 'System.Collections.Generic.List.this[int]'

My question is: Are they any alternative/better ways to manage this exception ?

Thanks.

EDIT: I got my answer, can you please close this question.

You should think of adding your stack trace and a bigger code sample of your issue ! — Kevin Avignon, Nov 28 '17 at 19:38
Oh, I'm sorry(First time posting). I thought it was clear, I'll edit my question right away. — RandomBeginner, Nov 28 '17 at 19:40
https://stackoverflow.com/questions/414981/directly-modifying-listt-elements. TDLR: don't use a struct. — Dylan Nicholson, Nov 28 '17 at 20:04
I'm sorry I've already read that post, but I'm not sure what do you mean by not using a struct, as I didn't use it? — RandomBeginner, Nov 28 '17 at 20:12
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score 1 · Accepted Answer · answered Nov 28 '17 at 20:22

Rather than working directly with Rectangles, how about adding a BodyPart class, with some manipulation methods:

public class BodyPart
{
    private Rectangle rectangle;

    public BodyPart(Rectangle rectangle)
    {
        this.rectangle = rectangle;
    }

    public void MoveY(int value)
    {
        rectangle.Y += value;
    }

    public void MoveX(int value)
    {
        rectangle.X += value;
    }
}

public class Snake
{
    public List<BodyPart> Parts = new List<BodyPart>();
}

public class AppSnake
{
    public void Run()
    {
        var snake = new Snake();
        snake.Parts.Add(new BodyPart(new Rectangle(760, 25, 8, 8)));
        snake.Parts[0].MoveY(-10);
    }
}

Thank you, this was really useful and I'm better using a class for the bodyparts. — RandomBeginner, Nov 28 '17 at 21:19

John Alexiou · Answer 2 · 2017-11-28T20:54:37.290

Rectangle, RectangleF, Point, PointF, Size and SizeF are value types (struct type). This means that you can not/should not change an individual part of the structure.

The reason is that unlike classes each variable keeps its own copy of the structure. When you type list[0].X = 10, the indexer list[0] returns a copy of the rectangle and not the original value. The correct way is to assign a new rectangle list[0] = A which copies all the values from A into the array.

Please read more on value types and structs before attempting to write code that uses them.

The quickest way to fix your code without completely changing it around is by adding methods that manipulate all the body parts in predefined ways:

public class Snake
{
    public List<Rectangle> Parts { get; } = new List<Rectangle>();

    public void MoveX(int delta)
    {
        for(int i = 0; i < Parts.Count; i++)
        {
            // Read the current rectangle from the list
            var rect = Parts[i];
            // Change the coordinate.
            rect.X += delta;
            // Write the modified rectangle back into the list
            Parts[i] = rect;
        }
    }
    public void MoveY(int delta)
    {
        for(int i = 0; i < Parts.Count; i++)
        {
            // Read the current rectangle from the list
            var rect = Parts[i];
            // Change the coordinate.
            rect.Y += delta;
            // Write the modified rectangle back into the list
            Parts[i] = rect;
        }
    }
}

Thank you so much, although I'm aware of structs in C#, it didn't occur to me that these objects are structs. But everything is clear now. — RandomBeginner, Nov 28 '17 at 21:21

score -1 · Answer 3 · answered Nov 28 '17 at 19:46

-1

Use Rectangle.Location and Rectanagle.Size properties e.g.:

snk.bodyParts[0].Location = new Point(newX, newY);
snk.bodyParts[0].Size = new Size(newWidth, newHeight);

answered Nov 28 '17 at 19:46

Ilya Hmurovich

27
6

I'm sorry but it didn't work, I already tried this before coming up with the solution in the post. – RandomBeginner Nov 28 '17 at 19:55

Cannot modify the return value of a collection of rectangles

3 Answers3