Extensible enums: Breaking change for .NET libraries that you need to be aware of

A while back I wrote a blog post describing the Extensible Enums - a new feature that is part of Dynamics 365 for Operations:
http://kashperuk.blogspot.com/2016/09/development-tutorial-extensible-base.html

I explained that when marking an enumeration as extensible, the representation of this enum under the hood (in CLR) changes. Here's the specific quote:

The extensible enums are represented in CLR as both an Enum and a Class, where each enum value is represented as a static readonlyfield. So accessing a specific value from the above enum, say, NumberSeqModule::Invent would under the hood look something like NumberSeqModule_Values.Invent, where Invent is of type NumberSeqModule which is an Enum. It would in turn call into AX to convert the specific named constant "Invent" to its integer enumeration value through a built-in function like the symbol2Value on DictEnum.

Something that was not super clear in the post is that this was actually a breaking change that might impact your .NET solutions relying on one of these base enumerations.

Problem statement

As part of enabling further extensibility in the application for the next release, we have made a number of additional base enums extensible.

Let's take enum TMSFeeType as an example. Assume we have made it extensible in X++. That means that in our C# project, where we use this enum, we will no longer be able to access it from Dynamics.AX.Application namespace by name, like so:

switch (accessorialFeeType)
{
    case TMSFeeType.Flat:
        // Do something
        break;

    case TMSFeeType.PerUOM:
        // Do something else
        break;
    // etc.
}

If you navigate to its definition, you will notice that the enum declaration is empty:

namespace Dynamics.AX.Application
{
    public enum TMSFeeType
    {
    }
}

The proper way to use the enum that is extensible is to reference the above mentioned class suffixed with _Values, which lives in the Dynamics.AX.Application.ExtensibleEnumValues namespace, like so:

if (accessorialFeeType == TMSFeeType_Values.Flat)
{
    // Do something
}
else if (accessorialFeeType == TMSFeeType_Values.PerUOM)
{
    // Do something else
}

Note: Because the values are now determined at runtime by going to the AOS and asking for the correct integer value of this enum, they cannot be used in a switch/case block, which expects constant expressions known at compile-time.

What's next

Obviously, this situation is not great. 

Let's hope that Microsoft will think of a good way to address this going forward.

Question to you

That leads to a question - how many of you actually have .NET libraries relying on application code in Dynamics 365 for Operations and might be impacted by us making some of the enums extensible in the next major release?