C# Casting with objects to Enums

Question

This question relates to casting of enums within generic methods

Given an enum

public enum Crustaceans
{
    Frog = 1,
    Toad = 4
}

I can create an instance of my enum simply enough

short val = 4;
Crustaceans crusty = (Crustaceans) val;

However, if

short val = 4;
object obj = (object) val;
Crustaceans crusty = (Crustaceans)obj;

a runtime exception is thrown attempting to perform the initialisation of crusty.

Can anyone explain why this is happening, and why it is not legal to do such a thing.

Not that I really wanted to do this, but I cam across an issue when trying to get something similar happening with generics and effectively that is what is happening under the covers. i.e.

public T dosomething<T>(short val) where T : new()
{
    T result = (T)(object) val;
    return result;
}

So what I am attempting to do is have a generic function that works with enums and non-enums (not so critical-but would be nice) that can be set to a short value without throwing an exception and actually initialising the correct enum value.

Answer 1

Something like this probably will help you:

public T dosomething<T>(object o) {    T enumVal= (T)Enum.Parse(typeof(T), o.ToString());    return enumVal; } 

But this will work only with enums, for clear reason of using Enum.Parse(..)

And use this like, for example:

object o = 4; dosomething<Crustaceans>(o); 

That will return Toad in your case.

Answer 2

In case of integral types boxed as objects the correct way to do the conversion is using Enum.ToObject method:

public T Convert<T>(object o) {    T enumVal= (T)Enum.ToObject(typeof(T), o);    return enumVal; } 

Answer 3

There are cases when you can not use Generics (like in a WPF Converter when you get the value as an object). In this case you can not cast to int because the enum type may not be an int. This is a general way to do it without Generics. The Example is given inside a WPF Converter, but the code inside is general:

using System;
using System.Windows;
using System.Windows.Data;

.
.
.

public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    var enumType = value.GetType();
    var underlyingType = Enum.GetUnderlyingType(enumType);
    var numericValue = System.Convert.ChangeType(value, underlyingType);
    return numericValue;
}