C# enum型安全

Question

是否有一种方法强迫C#枚举只接受几个显式命名常量中的一个，还是有另一个特性可以接受？C#引用有这样的事后考虑：

可以将任意整数值赋值给枚举类型。但是，您不应该这样做，因为隐含的期望是枚举变量将只保存枚举定义的一个值。将任意值赋值给枚举类型的变量将带来错误的高风险。

(设计了一种新的语言，允许这种草率。)这让我很困惑。)

Answer 1

据我所知，您无法阻止C#允许在枚举和整数之间进行转换。

作为解决办法，您可以使用带有受限实例化的自定义类型。用法将类似，但您也将有机会为这种类型定义方法和运算符。

就说你有这件事：

enum DayOfWeek
{
    Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday 
}

你可以用一个密封的类代替它。优点是免费获得比较(因为在这种情况下，值比较和引用比较是等价的)。缺点是(与C#中的所有引用类型一样)，它是可空的。

sealed class DayOfWeek
{
    public static readonly DayOfWeek Monday = new DayOfWeek(0);
    public static readonly DayOfWeek Tuesday = new DayOfWeek(1);
    public static readonly DayOfWeek Wednesday = new DayOfWeek(2);
    public static readonly DayOfWeek Thursday = new DayOfWeek(3);
    public static readonly DayOfWeek Friday = new DayOfWeek(4);
    public static readonly DayOfWeek Saturday = new DayOfWeek(5);
    public static readonly DayOfWeek Sunday = new DayOfWeek(6);

    private readonly int _value;

    private DayOfWeek(int value) 
    {
        _value = value;
    }
}

或者你可以用一个结构。它的优点是它不能为空，因此它甚至更类似于枚举。缺点是您必须手动实现比较代码：

struct DayOfWeek
{
    public static readonly DayOfWeek Monday = new DayOfWeek(0);
    public static readonly DayOfWeek Tuesday = new DayOfWeek(1);
    public static readonly DayOfWeek Wednesday = new DayOfWeek(2);
    public static readonly DayOfWeek Thursday = new DayOfWeek(3);
    public static readonly DayOfWeek Friday = new DayOfWeek(4);
    public static readonly DayOfWeek Saturday = new DayOfWeek(5);
    public static readonly DayOfWeek Sunday = new DayOfWeek(6);

    private readonly int _value;

    private DayOfWeek(int value)
    {
        _value = value;
    }

    public bool Equals(DayOfWeek other)
    {
        return _value == other._value;
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj))
        {
            return false;
        }
        return obj is DayOfWeek && Equals((DayOfWeek)obj);
    }

    public override int GetHashCode()
    {
        return _value;
    }

    public static bool operator ==(DayOfWeek op1, DayOfWeek op2)
    {
        return op1.Equals(op2);
    }

    public static bool operator !=(DayOfWeek op1, DayOfWeek op2)
    {
        return !(op1 == op2);
    }
}

Answer 2

将任何整数强制转换为枚举的能力主要是由于性能原因，但作为值类型实现的枚举不能保护其不包含未定义的值。考虑一种类似于：

public enum Condition {
  Right = 1,
  Wrong = 2
}

即使枚举变量的赋值仅限于定义的值，您仍然可以通过简单地将其放入类中来创建未定义的值：

public class Demo {
  public Condition Cond;
}

当创建类的实例时，成员被初始化为零，因此Cond成员变量将具有未定义的(Condition)0值。

您可以创建一个包装类，以确保枚举值在定义的值中：

public sealed class SafeEnum<T> where T : struct {

  public T Value { get; private set; }

  public SafeEnum(T value) {
    if (!(value is Enum)) {
      throw new ArgumentException("The type is not an enumeration.");
    }
    if (!Enum.IsDefined(typeof(T), value)) {
      throw new ArgumentException("The value is not defined in the enumeration.");
    }
    Value = value;
  }

}

示例：

var cond = new SafeEnum<Condition>(Condition.Right); // works
Condition d = cond.Value;

var cond = new SafeEnum<int>(42); // not an enum

var cond = new SafeEnum<Condition>((Condition)42); // not defined

类的实例只能包含在枚举中定义的值，否则构造函数将不允许创建实例。

由于类是不可变的，因此值不能更改为未定义的值。