如果我有这样的枚举:

enum Beer
{
    Bud = 10,
    Stella = 20,
    Unknown
}

为什么在将这些值之外的 int 转换为 Beer 类型时不抛出异常？

例如下面的代码没有抛出异常，它向控制台输出'50':

int i = 50;
var b = (Beer) i;

Console.WriteLine(b.ToString());

我觉得这很奇怪……谁能解释一下？

最佳答案

取自Confusion with parsing an Enum

这是创建 .NET 的人的决定。枚举由另一种值类型(int、short、byte 等)支持，因此它实际上可以具有任何有效值对于那些值类型。

我个人不喜欢这种工作方式，所以我制作了一系列实用方法:

/// <summary>
/// Utility methods for enum values. This static type will fail to initialize 
/// (throwing a <see cref="TypeInitializationException"/>) if
/// you try to provide a value that is not an enum.
/// </summary>
/// <typeparam name="T">An enum type. </typeparam>
public static class EnumUtil<T>
    where T : struct, IConvertible // Try to get as much of a static check as we can.
{
    // The .NET framework doesn't provide a compile-checked
    // way to ensure that a type is an enum, so we have to check when the type
    // is statically invoked.
    static EnumUtil()
    {
        // Throw Exception on static initialization if the given type isn't an enum.
        Require.That(typeof (T).IsEnum, () => typeof(T).FullName + " is not an enum type.");
    }

    /// <summary>
    /// In the .NET Framework, objects can be cast to enum values which are not
    /// defined for their type. This method provides a simple fail-fast check
    /// that the enum value is defined, and creates a cast at the same time.
    /// Cast the given value as the given enum type.
    /// Throw an exception if the value is not defined for the given enum type.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="enumValue"></param>
    /// <exception cref="InvalidCastException">
    /// If the given value is not a defined value of the enum type.
    /// </exception>
    /// <returns></returns>
    public static T DefinedCast(object enumValue)

    {
        if (!System.Enum.IsDefined(typeof(T), enumValue))
            throw new InvalidCastException(enumValue + " is not a defined value for enum type " +
                                           typeof (T).FullName);
        return (T) enumValue;
    }

    /// <summary>
    /// 
    /// </summary>
    /// <param name="enumValue"></param>
    /// <returns></returns>
    public static T Parse(string enumValue)
    {
        var parsedValue = (T)System.Enum.Parse(typeof (T), enumValue);
        //Require that the parsed value is defined
        Require.That(parsedValue.IsDefined(), 
            () => new ArgumentException(string.Format("{0} is not a defined value for enum type {1}", 
                enumValue, typeof(T).FullName)));
        return parsedValue;
    }

    public static bool IsDefined(T enumValue)
    {
        return System.Enum.IsDefined(typeof (T), enumValue);
    }

}


public static class EnumExtensions
{
    public static bool IsDefined<T>(this T enumValue)
        where T : struct, IConvertible
    {
        return EnumUtil<T>.IsDefined(enumValue);
    }
}

这样，我可以说:

if(!sEnum.IsDefined()) throw new Exception(...);

...或者:

EnumUtil<Stooge>.Parse(s); // throws an exception if s is not a defined value.

编辑

除了上面给出的解释之外，您必须意识到 Enum 的 .NET 版本遵循的模式更多地是受 C 启发的模式，而不是受 Java 启发的模式。这使得 "Bit Flag" enums 成为可能它可以使用二进制模式来确定特定“标志”是否在枚举值中处于事件状态。如果您必须定义所有可能的标志组合(即 MondayAndTuesday、MondayAndWednesdayAndThursday)，这些将非常乏味。因此，能够使用未定义的枚举值会非常方便。当您希望在不利用这些技巧的枚举类型上实现快速失败行为时，它只需要做一些额外的工作。

关于c# - 为什么将 int 强制转换为无效的枚举值不会引发异常？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/6413804/