Friday, 18 October 2013

Start C# Enumerations at 1

Coming from a background in C/C++ I was brought up with the problems of uninitialised variables, which, through the use of better compiler warnings and static code analysis tools I managed to keep under control. Sadly uninitialised member variables are still the domain of the more specialist tools as far as I know. With C# the problem still exists, to some extent. Whilst C#, nay .Net, guarantees that variables and class fields will be initialised to a known value (0 or the moral equivalent, e.g. false, null, etc.) that does not always help in the fight against bugs due to uninitialised variables [1].


In C#, unlike C++, you have Reflection which pretty much allows you to do whatever you like and ignore whatever access controls and constraints the class designer put in place by poking data directly into class fields. When you create an object via reflection it only initialises values to their defaults, there must be some other protocol in place to allow a post-initialisation method to be called, e.g. the [OnDeserialized] attribute.


With the enum type essentially being implemented as a primitive, an integer, it gets the value 0 after construction by reflection. Whereas a real integer always has the value 0 as part of its range, an enum may not - it’s usually constrained to a small subset of the entire range of integral values. What this means in practice is that the bit pattern for 0 may or may not fall onto one of the values in the enumeration depending on how you defined it.

enum Colour
  Red,   // first is 0 by default

The C# documentation suggests that you make “the default value" of your enumeration the first. What they are really saying though is that you should try and ensure that an uninitialised variable of your enumeration type happens to coincide with a valid value, and therefore a value that will cause the code to behave sensibly.

Maybe, I’ve being doing C++ for too long because frankly that advice disturbs me.

Special Values

I’m sure there are cases where a default value might makes sense in some enumerations but I’ve rarely come across a case where it does [2]. Also I thought we’d got passed the whole “special” value problem and put the likes of -1, NaN and NULL behind us? If the value is optional, then use an optional-style type, like Nullable<T>. Only, it’s not actually optional, it’s just optional when you want to create the object via reflection [3].

Don’t Mask The Error

So, what’s the alternative? Well you could start your enumeration from 1 instead of 0, by being explicit on the first value:-

enum Colour
  Red = 1,

At least now you can detect an uninitialised value when you come to process a value from the enumeration because both the static variation through a switch statement:-

switch (colour)
  case red: . . .
  case white: . . .
  case blue: . . .
  default: throw new ArgumentOutOfRangeException();

…and the dynamic variation via a dictionary:-

var map = new Dictionary<Colour, Method>();
. . .
if (!map.TryGetValue(colour, out method))
  throw new ArgumentOutOfRangeException();

…allow a check to be made and an invalid value to be detected. Also don’t forget that you can use the Enum.IsDefined() static method to validate an integer value before casting it to its enum equivalent. In fact this kind of thing is just perfect for an extension method:-

public static T AsEnum<T>(this int value)
  where T : struct
  if (!typeof(T).IsEnum)
    throw new ArgumentException();

  if (!Enum.IsDefined(typeof(T), value))
    throw new ArgumentOutOfRangeException();

  return (T)Enum.ToObject(typeof(T), value);
. . .
int value = 1;
Colour colour = value.AsEnum<Colour>();
Assert.That(colour, Is.EqualTo(Colour.Red));

Sadly C# generics aren’t quite good enough to constrain the type specifically to an enum, a value type is the best we can do at compile time, hence the runtime check at the start of the method. Also I’d be wary of the performance of the Enum.ToObject() to perform the cast as it seems a very ham-fisted way of just doing a (T)value style cast. As always there’s plenty more depth to this topic on StackOverflow.

Tests - Simples

I know the stock answer to my paranoia is just to write some unit tests and make sure the correct behaviour is covered, but there still feels like there is a subtlety here that is just waiting to trick up the unwary maintenance programmer down the line if you go with the party line.


[1] My definition of “uninitialised” is based on a variable not having the value it was intended to have, that means it has to be a conscious decision to allow a variable to take on the default value, not an unconscious one. 

[2] Using an enumeration to define a set of bitfield flags is possibly one case. But that in itself is a special type of enumeration that is hardly different from an integer and a class of constants. An enumerated type is commonly used as a type identifier which is a different sort of beast to a set of boolean flags.

[3] I’m currently working with a web API and MongoDB back-end and all the marshalling is being handled automatically via reflection, which is all new ground for me.

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