やべｗ間違ってたｗｗ

採用試験に挑戦 - 気楽に諸々
↑検証はあえて投稿後にやったけど、案の定間違ってたｗ
僕がホントにやりたかったことはこっちのコード

class Position
{
    public int X { get; set; }
    public int Y { get; set; }

    public void FitToGrid(int gridPitch)
    {
        this.X = this.CalculateFitPoint(this.X, gridPitch);
        this.Y = this.CalculateFitPoint(this.Y, gridPitch);
    }

    private int CalculateFitPoint(int point, int gridPitch)
    {
        int fraction = point % gridPitch;
        bool truncate = (fraction * 2 < gridPitch);
        return (truncate) ? (point - fraction) : (point + (gridPitch - fraction));
    }
}