Question

I built a extension method for Enums (Enumerations) – , name it, say GetEnumSecondName

static string GetEnumSecondName(this Enum myEnumInstance)    {...}

Now, I have a generic method, that should take a Enumeration and return all the second names for that type.

List<string> GetSecondNames<T : ?T:Enum ? >()
{ 
  // ...

  foreach T member in GetAllMembers<T>()
    // should work only for Enum instances
    resultList.Add(member.GetEnumSecondName()); 

  // ...
}

Is there a workaround to do it?

Edit:

As I understood (thanks to Jon Skeet), C# does not support thins kind of constraint. If there are any VB.NET expert to confirm that “ordinary” VB.NET does not support it either. Thanks.

Answer 1

Yes, there’s a workaround. You may not like it though. You have to rewrite the IL to express the constraint you want – because the CLR allows it, but C# doesn’t. (The compiler respects the constraint; it just doesn’t let you express it in C# code.)

I have a project called Unconstrained Melody which does exactly this, introduced in a blog post.

It’s regrettable that you can’t express this, and maybe it’ll be fixed in a future version of the language. For now, IL rewriting is all there is as far as I know.

EDIT: I’ve just tried the constraint you’d want in VB:

Foo(Of T As { System.Enum, Structure }) (...)

And the compiler complains with:

error BC32061: 'Enum' cannot be used as a type constraint.

So no, you can’t do it in VB either. Oddly enough, the web page about that error doesn’t mention the restriction…

EDIT: To anyone wanting to play with Unconstrained Melody, there are a few steps required to get it working:

• You need to make sure you have an appropriate SDK directory as referred to by ConstraintChanger\Program.cs. In particular, check in \Program Files\Microsoft SDKs\Windows to see what version you’ve got – and change Program.cs appropriately
• Critically, you need a directory called “Rewritten” at the top level (i.e. alongside lib)
• If you’re using VS2010 you’ll need to go through the project upgrade at the start

Once all of that is correct, you should just be able to hit Ctrl-Shift-B and get a working build. Do not remove and replace the project references – the test assembly needs to refer to the rewritten one, not the project it’s created from.

I’ll attempt to address some of these issues tonight – and possibly even create a Nuget package…