C# Span2D CopyTo 未正确复制二维范围

Question

I'm trying to use the Span2D type to "roll" entries in a 2d array, but it's not working as expected.

By rolling I mean the following - given an array such as:

{
    { 1, 1, 1, 1, 1 },
    { 2, 2, 2, 2, 2 },
    { 0, 0, 0, 0, 0 },
}

I would like to copy the first two rows down one row, so the top row can be repopulated. After the roll operation the array should look like this:

{
    { 1, 1, 1, 1, 1 },
    { 1, 1, 1, 1, 1 },
    { 2, 2, 2, 2, 2 },
}

The Span2d CopyTo method seems perfect for this - I create a Slice of the top two rows, and a slice of the bottom two rows, copy the first slice to the second slice. But instead of the expected result above, I get:

{
    { 1, 1, 1, 1, 1 },
    { 1, 1, 1, 1, 1 },
    { 1, 1, 1, 1, 1 },
}

Here's a runnable class that shows the problem:

public class SpanTest
{
    public static void Main()
    {
        int[,] array =
        {
            { 1, 2, 3, 4, 5 },
            { 0, 0, 0, 0, 0 },
            { 0, 0, 0, 0, 0 },
        };

        var h = array.GetLength(0) - 1;
        var w = array.GetLength(1);
        Console.WriteLine($"slice height:{h} width: {w}\n-----------");

        Span2D<int> span = array;
        Console.WriteLine($"{span.ToStringMatrix()}-----------");
        
        var sourceSlice = span.Slice(0, 0, h, w);
        Console.WriteLine($"{sourceSlice.ToStringMatrix()}-----------");

        var targetSlice = span.Slice(1, 0, h, w);
        Console.WriteLine($"{targetSlice.ToStringMatrix()}-----------");
        
        sourceSlice.CopyTo(targetSlice);
        Console.WriteLine($"{span.ToStringMatrix()}-----------");

    }
}

with a helper for printing the Span2D s:

public static class Utils
{
    public static string ToStringMatrix<T>(this Span2D<T> arr)
    {
        var sb = new StringBuilder();
        for (var i = 0; i < arr.Height; i++)
        {
            for (var j = 0; j < arr.Width; j++)
            {
                sb.Append($"{arr[i, j]} ");
            }

            sb.Append(Environment.NewLine);
        }

        return sb.ToString();
    }
}

How can I make the copy operation behave as expected? Thanks

Answer 1

Well, the answer is quite obvious actually - at least it was when it occurred to me at 5am this morning!

Span2D wraps an array, copying to itself alters the backing array during the copy process. By the time the second row is copied, it already contains the contents of the first row. And hence, the first row gets propagated throughout the 2d array.