C# 枚举和可能有一个默认值？

Question

所以我有以下枚举，我需要它有一个由其他代码使用的“无”项，如方法：GetCompassDirectionFromUnitVector，如下所示：

    public enum CompassDirection
{
    North = 90,
    Northeast = 45,
    East = 0,
    Southeast = -45,
    South = -90,
    Southwest = -135,
    West = 180,
    Northwest = 135,
    None = null
}


/// <summary>
/// Returns compass direction (North, Southwest, etc.) from a given direction unit vector. 
/// Returns CompassDirection.None if given Vector is not a unit vector.
/// </summary>
/// <param name="vec">Direction unit vector</param>
/// <returns></returns>
public static CompassDirection GetCompassDirectionFromUnitVector(Vector vec)
{
    Dictionary<Vector, CompassDirection> list = new Dictionary<Vector, CompassDirection>()
    {
        {new Vector(0, 0), CompassDirection.None},
        {new Vector(0, 1), CompassDirection.North},
        {new Vector(0, -1), CompassDirection.South},
        {new Vector(1, 0), CompassDirection.East},
        {new Vector(-1, 0), CompassDirection.West},
        {new Vector(1, 1), CompassDirection.Northeast},
        {new Vector(1, -1), CompassDirection.Southeast},
        {new Vector(-1, 1), CompassDirection.Northwest},
        {new Vector(-1, -1), CompassDirection.Southwest},
    };
    if (list.ContainsKey(vec))
    {
        return list[vec];
    }
    return CompassDirection.None;
}

但是我写的是下面的方法：

   Angle GetAngleFromCompassDirection(CompassDirection dir)

因此，在枚举值中包含角度在这里非常有用。 但是整数不能为空，因此具有“无”值枚举是无效的。

我该如何解决这个问题？

Answer 1

枚举值的默认值始终为零 (0)。我建议您将实现更改为：

public static CompassDirection? GetCompassDirectionFromUnitVector(Vector vec)
{
    Dictionary<Vector, CompassDirection> list = new Dictionary<Vector, CompassDirection>()
    {
        {new Vector(0, 0), CompassDirection.None},
        {new Vector(0, 1), CompassDirection.North},
        {new Vector(0, -1), CompassDirection.South},
        {new Vector(1, 0), CompassDirection.East},
        {new Vector(-1, 0), CompassDirection.West},
        {new Vector(1, 1), CompassDirection.Northeast},
        {new Vector(1, -1), CompassDirection.Southeast},
        {new Vector(-1, 1), CompassDirection.Northwest},
        {new Vector(-1, -1), CompassDirection.Southwest},
    };
    if (list.ContainsKey(vec))
    {
        return list[vec];
    }
    return null;
}

（注意 Nullable 返回类型）然后您可以像对待 CompassDirection.None 一样对待空值。