Decorating Partial Class Members in C#
Posted by bsstahl on 2011-03-08 and Filed Under: development
Anyone who uses code generation more than trivially in their .NET solutions knows how difficult it is to add metadata to a member of a generated class, even if that class is marked as partial. Partial classes are terrific for adding members to generated classes, and even allow you to change metadata and other information about the class itself, such as by adding an interface that it implements. You cannot however, modify an existing member of the class by marking it to not be serialized, or to make it a part of a WCF data contract. I believe, as more and more code is generated by .NET developers using Entity Framework, wsdl.exe and other proxy generation tools, or even T4 directly; we will see a greatly increased need for a workable solution to this problem.
The most common work-around for this problem today is the use of “buddy classes” where a separate class is created to hold the metadata. A good example of this type of solution can be found in ASP.NET Dynamic Data. However, as you might imagine, this type of implementation has some fairly severe drawbacks. For one, it requires some sort of mechanism for unifying the functionality of the two classes. An example of this is the case where we have a property on an Entity Framework POCO that should not be serialized when we serialize the rest of the class. We cannot serialize the POCO correctly unless we use a custom serializer that is aware of the metadata in the buddy class. We have similar problems if we are trying to use a generated base class as the output of a WCF service. If the [DataContract] attribute is not already on the generated code (which requires a reference to the System.Runtime.Serialization library) we will have to create a separate object that implements the DataContract to be exposed by the service. In most cases, this object will be an almost exact duplicate of the state features of the original object, forcing the replication of some code, even if that code is just the properties of the object.
There is a feature that has been proposed in the C# language for some time now that would allow this type of behavior within a single class. This feature would use the keyword existing to allow the duplication of the definition of partial class members into non-code-generated files so that those members can be decorated with additional metadata without risk of being overwritten when the automatic code is regenerated, and without duplicating the functionality in the member. Unfortunately, it appears that this feature is not going to make it into the next version of C# and is at risk of not being in the following version as well. In an email exchange, Mads Torgersen, C# Program Manager at Microsoft, stated that
…maybe we’re looking at C# next-next-next before the next big set of new feature ads makes it in [to the language].
One of the reasons (although certainly not the only reason) for this delay, as explained by Mads, is what really struck me:
To be honest the clamoring for the “existing” feature has died down quite a bit. I think that people are probably resigning to the use of “buddy classes” for this purpose; this seems to be the most common substitute.
…ultimately your feedback is at the basis of how we make these choices.
So I began to wonder, have we really resigned ourselves to using these buddy classes? Are they less of a problem for others then they are for me? Am I, perhaps, missing a fairly easy solution to this problem that involves the use of “buddy classes” but solves the issues with that methodology? Or perhaps, are there other features that we want in the language that are just more important to us than this one?
Please let me know if am I missing something or if there are others out there who feel as I do on this feature request. If there are many others like me, perhaps we can put our voices together to, as Mads suggests, let Microsoft know what we need.