Can I foreach over an array of structs without copying the elements in C# 8?

Question

With the new readonly instance member features in C# 8, I try to minify unnecessary copying of struct instances in my code.

I do have some foreach iterations over arrays of structs, and according to this answer, it means that every element is copied when iterating over the array.

I thought I can simply modify my code now to prevent the copying, like so:

// Example struct, real structs may be even bigger than 32 bytes.
struct Color
{
    public int R;
    public int G;
    public int B;
    public int A;
}

class Program
{
    static void Main()
    {
        Color[] colors = new Color[128];
        foreach (ref readonly Color color in ref colors) // note 'ref readonly' placed here
            Debug.WriteLine($"Color is {color.R} {color.G} {color.B} {color.A}.");
    }
}

This sadly does not compile with

CS1510  A ref or out value must be an assignable variable

However, using an indexer like this compiles:

static void Main()
{
    Color[] colors = new Color[128];
    for (int i = 0; i < colors.Length; i++)
    {
        ref readonly Color color = ref colors[i];
        Debug.WriteLine($"Color is {color.R} {color.G} {color.B} {color.A}.");
    }
}

Is my syntax in the foreach alternative wrong, or is this simply not possible in C# 8 (possibly because of how the enumeration is implemented internally)? Or is C# 8 applying some intelligence nowadays and does no longer copy the Color instances by itself?

Answer 1

foreach works based on the target type's definitions rather than some internal blackboxes. We could make use of this to create by-ref enumeration support:

//using System;

public readonly struct ArrayEnumerableByRef<T>
{
    private readonly T[] _target;

    public ArrayEnumerableByRef(T[] target) => _target = target;

    public Enumerator GetEnumerator() => new Enumerator(_target);

    public struct Enumerator
    {
        private readonly T[] _target;

        private int _index;

        public Enumerator(T[] target)
        {
            _target = target;
            _index = -1;
        }

        public readonly ref T Current
        {
            get
            {
                if (_target is null || _index < 0 || _index > _target.Length)
                {
                    throw new InvalidOperationException();
                }
                return ref _target[_index];
            }
        }

        public bool MoveNext() => ++_index < _target.Length;

        public void Reset() => _index = -1;
    }
}

public static class ArrayExtensions
{
    public static ArrayEnumerableByRef<T> ToEnumerableByRef<T>(this T[] array) => new ArrayEnumerableByRef<T>(array);
}

Then we could enumerate an array with foreach loop by reference:

static void Main()
{
    var colors = new Color[128];

    foreach (ref readonly var color in colors.ToEnumerableByRef())
    {
        Debug.WriteLine($"Color is {color.R} {color.G} {color.B} {color.A}.");
    }
}