Indexer that points to item in struct array don't work

Question

I have a class called Surface, in this class i have an array of type struct Color.

public class Surface
{
    private Color[,] pixels;

    public Color this[int x, int y]
    {
        get { return pixels[y, x]; }
    }
}

[StructLayout(LayoutKind.Explicit)]
public struct Color
{
    [FieldOffset(0)]
    public byte R;

    public void Set(byte r)
    {
        R = r;
    }
}

However when i try to access the color using the indexer it don't get updated.

mySurface[x, y].Set(255); // Will not work, i don't get an error but the color don't get updated.

How can i solve this problem?

You've just discovered why [mutable structs are evil](http://stackoverflow.com/questions/441309/why-are-mutable-structs-evil) Try `var temp = mySurface[x,y]; temp.Set(255); mySurface[x,y] = temp;` instead. — juharr, Apr 06 '17 at 12:08
Other question connected http://stackoverflow.com/q/4720475/613130 — xanatos, Apr 06 '17 at 12:08

Jon Skeet · Accepted Answer · 2017-04-06T14:26:15.830

How can i solve this problem?

Well you could avoid creating mutable structs and exposing public fields, to start with. That's where the problem is coming from. Your code is effectively:

Color tmp = mySurface[x, y]; // Take a copy from the array...
tmp.Set(255); // This only affects the copy

To change the array, you'll need to call the setter on the indexer instead. For example:

Color tmp = mySurface[x, y];
tmp.Set(255);
mySurface[x, y] = tmp;

Assuming you actually have several values in your struct, it would be simpler if you'd make your struct immutable but provide methods that returned new values, just like DateTime.AddDays etc. Then you could write code like:

mySurface[x, y] = mySurface[x, y].WithRed(255);

Options if you really want to avoid using a setter:

Use ref return from C# 7: redefine your indexer to return a ref Color; although then you can't have a setter.
Make Color a class instead of a struct
Use a reference type inside Color, so you don't need to change the bits in the Color value itself. (This is really nasty - I'm not suggesting that.)

side question: new ValueTuple struct both exposes public fields and is mutable. How so? Some perfomance considerations? — Evk, Apr 06 '17 at 12:14
You also can avoid calling setter, if you return `Color` by reference: `public ref Color this[int x, int y] => ref pixels[y, x];`. — user4003407, Apr 06 '17 at 13:52

score 0 · Answer 2 · edited May 23 '17 at 12:17

0

Structs are value types, so if you get it from the array by calling pixels[y,x], you will actually create a copy of the struct to change the field on.

See also Unable to Modify struct Members

edited May 23 '17 at 12:17

Community

1
1

answered Apr 06 '17 at 12:10

Fortega

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It's not changing the property that creates the copy (not that there *is* a property in `Color`) - it's the indexer. – Jon Skeet Apr 06 '17 at 12:10
You are right, Jon. I didn't explain that well... I edited the answer to be more correct. – Fortega Apr 06 '17 at 12:15

Indexer that points to item in struct array don't work

2 Answers2