複数のキーで比較を行うComparer

KeyComparer.cs

// OrderBy(x => x.Id).ThenBy(x => x.Name) みたいな比較をするComparer
// var comparer = KeyComparer<Foo>.CreateBy(x => x.Id).ThenBy(x => x.Name); みたいに使う
public class KeyComparer<T> : IComparer<T>
{
    readonly List<Comparison<T>> comparisons_;
    private KeyComparer(params Comparison<T> []comparisons)
    {
        comparisons_ = comparisons.ToList();
    }

    public int Compare(T x, T y)
    {
        var ret = 0;
        foreach (var c in comparisons_)
        {
            ret = c(x, y);
            if (ret != 0)
                break;
        }
        return ret;
    }

    public static KeyComparer<T> CreateBy<TKey>(Func<T, TKey> selector)
        where TKey: IComparable
    {
        return new KeyComparer<T>((T x, T y) => Comparer<TKey>.Default.Compare(selector(x), selector(y)));
    }

    public KeyComparer<T> ThenBy<TKey>(Func<T, TKey> selector)
        where TKey: IComparable
    {
        // イミュータブルにして別のインスタンスを返すようにした方がいいかも
        comparisons_.Add((T x, T y) => Comparer<TKey>.Default.Compare(selector(x), selector(y)));
        return this;
    }
}