ちろちろのパクり

CharacterArt.cs

// using System.Collections.Generic;

public static class CharacterArt
{
    public static IEnumerable<Item> Diamond(int size)
    {
        for (int i = 0; i < size; i++)
        {
            yield return new Item
            {
                Space = (size - i) - 1,
                Symbol = (i + 1) * 2 - 1
            };
        }

        for (int i = 0; i < size - 1; i++)
        {
            yield return new Item
            {
                Space = i + 1,
                Symbol = (size - i) * 2 - 3
            };
        }
    }
}

Item.cs

public class Item
{
    public int Space { get; set; }
    public int Symbol { get; set; }
}