混淆解析一个枚举 [英] Confusion with parsing an Enum

问题描述

我正在将一个字符串类型的数值转换成相应的枚举。当我测试我的代码时，我发现有趣的行为让我感到困惑。

I am converting a numeric value that is of string type into a corresponding Enum. While I was testing my code, I discovered interesting behavior that has me confused.

使用下面的代码示例，有人可以明白为什么如果/当s变量的值不匹配一个值时，不会抛出异常的枚举值？另外，如何将sEnum var设置为在Stooge枚举的定义中不存在的值？

Using the code example below, can somebody shed light on why an exception isn't thrown if/when the "s" variable has a value that doesn't match one of the Enum values? Also, how is that the sEnum var can be set to a value that doesn't exist within the definition of the Stooge enum?

class Program
{
    enum Stooge
    {
        Unspecified,
        Moe,
        Larry,
        Curly,
        Shemp
    }

    static void Main(string[] args)
    {
        while (true)
        {
            Console.WriteLine("Enter a number...");

            string s = Console.ReadLine();
            Stooge sEnum = (Stooge)(int.Parse(s)); //Why doesn't this line throw if s != 0, 1, 2, 3, or 4?

            Console.WriteLine("\r\nYou entered: {0}\r\nEnum String Value: {1}\r\nEnum Int Value: {2}\r\n", s, sEnum.ToString(), (int)sEnum);
        }
    }
}

推荐答案

这是创建.NET的人的一个决定。枚举由另一个值类型（ int ， short ， byte 等），因此它实际上可以具有对这些值类型有效的任何值。

This was a decision on the part of the people who created .NET. An enum is backed by another value type (int, short, byte, etc), and so it can actually have any value that is valid for those value types.

我个人不是这个工作方式的粉丝，所以我做了一系列的实用方法：

I personally am not a fan of the way this works, so I made a series of utility methods:

/// <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);
    }

}

这样我可以说：

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

...或：

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

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