Confusion with parsing an Enum

Question

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.

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);
        }
    }
}

Answer 1

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.

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);
    }

}

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

This way, I can say:

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

... or:

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

Update

As Brandon Kramer pointed out in the comments, C# 7.3 introduced some new generic types that allow the where T : struct, IConvertible above to be replaced with where T : Enum, to get better compile-time checking of the Enum-ness of the type being passed in. That way you can get rid of the guard statement in EnumUtil's static constructor.

Answer 2

An enum is just technically an int (or whatever you have defined the enum's underlying type to be). you can check for a corresponding value in the enum, though with a call to Enum.IsDefined. More info here: Cast int to enum in C#