Question

I’m trying to use the code from this article: Creating Fake Enums, but I can’t figure out why it doesn’t work.

This code:

Console.WriteLine(FakeEnum.One.FriendlyName);
Console.WriteLine(FakeEnum.Four.FriendlyName);

generates an exception:

System.TypeInitializationException was unhandled
  Message="The type initializer for 'FakeEnum' threw an exception."
  Source="FakeEnum1"
  TypeName="FakeEnum"
  StackTrace:
       at FakeEnum1.Program.Main(String[] args) in ..\Test\FakeEnum1\Program.cs:line 26
       at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.NullReferenceException
       Message="Object reference not set to an instance of an object."
       Source="FakeEnum1"
       StackTrace:
            at FakeEnum.op_Equality(FakeEnum a, FakeEnum b) in ..\Test\FakeEnum1\FakeEnum.cs:line 158
            at FakeEnum.ToString(String format) in ..\Test\FakeEnum1\FakeEnum.cs:line 31
            at FakeEnum.ToString() in ..\Test\FakeEnum1\FakeEnum.cs:line 25
            at FakeEnum..ctor(Int32 value, String friendlyName) in ..\Test\FakeEnum1\FakeEnum.cs:line 171
            at FakeEnum..ctor(Int32 value) in ..\Test\FakeEnum1\FakeEnum.cs:line 165
            at FakeEnum..cctor() in ..\Test\FakeEnum1\FakeEnum.cs:line 13

If I comment the members declared with the (int) constructor, everything else works:

public static readonly FakeEnum One = new FakeEnum(1, "One's Friendly Name");
public static readonly FakeEnum Two = new FakeEnum(2, "Two's Friendly Name");
public static readonly FakeEnum Three = new FakeEnum(3, "Three's Friendly Name");
//public static readonly FakeEnum Four = new FakeEnum(4);
//public static readonly FakeEnum Five = new FakeEnum(5);
//public static readonly FakeEnum Six = new FakeEnum(6);

Now, if I make the constructors public, the following code works just fine:

FakeEnum a = new FakeEnum(14, "1 4");
FakeEnum b = new FakeEnum(28);
Console.WriteLine(a.FriendlyName);
Console.WriteLine(b.FriendlyName);

I just run out of ideas – what am I missing, and what generates the exception when using the original code?

Answer 1

It’s this bit:

FakeEnum temp = staticField.GetValue(null) as FakeEnum;
if(temp == null) continue;

That “==” is calling the == operator, which doesn’t expect the left hand side to be null:

public static bool operator == (FakeEnum a, FakeEnum b) { 
    return a.Equals(b);
}

You can fix the operator like this:

public static bool operator == (FakeEnum a, FakeEnum b) { 
    if (object.ReferenceEquals(a, b)) {
        return true;
    }
    if (object.ReferenceEquals(a, null) || object.ReferenceEquals(b, null)) {
        return false;
    }
    return a.Equals(b);
}

(Or by using object.Equals, as pointed out in another answer – doh!)

or you could change it to make Equals delegate to == instead of the other way round.

Personally I’d say this class is looking a tad hairy. It’s not even clear to me why == and != need overloading or indeed why Equals needs overriding unless you really have two separate objects with the same value (e.g. due to serialization, which is going to be icky anyway).