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[英]Is there a name for this pattern? (C# compile-time type-safety with “params” args of different types)
[英]C# enum type-safety
有没有办法强制C#枚举只接受几个明确命名的常量之一,还是有另一个功能呢? C#参考有了这个想法:
可以将任意整数值分配给枚举类型。 但是,您不应该这样做,因为隐含的期望是枚举变量只保存枚举定义的值之一。 将任意值分配给枚举类型的变量是为了引入高风险的错误。
(一种新的语言设计允许这种邋。。这让我感到困惑。)
据我所知,你不能阻止C#允许枚举和整数之间的转换。
作为一种变通方法,您可以使用具有受限实例化的自定义类型。 用法看起来类似,但您也可以为此类型定义方法和运算符。
说你有这个枚举:
enum DayOfWeek
{
Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
}
您可以在其位置使用密封类。 优点是您可以免费进行比较(因为在这种情况下,值比较和参考比较是等效的)。 缺点是(像C#中的所有引用类型一样),它可以为空。
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;
}
}
或者您可以使用结构。 它的优点是它不可空,因此它更类似于枚举。 缺点是你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);
}
}
将任何整数强制转换为枚举的能力主要是出于性能原因,但实现为值类型的枚举无法保护不包含未定义的值。 考虑一个枚举如:
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在类中来创建未定义的值:
public class Demo {
public Condition Cond;
}
创建类的实例时,成员初始化为零,因此Cond
成员变量将具有未定义的值(Condition)0
。
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;
}
}
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
该类的实例只能包含枚举中定义的值,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。
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