Are C#'s partial classes bad design? [closed]

Question

I'm wondering why the 'partial class' concept even exists in C#/VB.NET. I'm working on an application and we are reading a (actually very good) book relavant to the development platform we are implementing at work. In the book, the author provides a large code base/wrapper around the platform API and explains how he developed it as he teaches different topics about the platform development.

Anyway, long story short - he uses partial classes, all over the place, as a way to fake multiple inheritance in C# (IMO). Why he didn't just split the classes up into multiple ones and use composition is beyond me. He will have 3 'partial class' files to make up his base class, each w/ 3-500 lines of code... And does this several times in his API.

Do you find this justifiable? If it were me, I'd have followed the S.R.P. and created multiple classes to handle different required behaviors, then created a base class that has instances of these classes as members (e.g. composition). Why did MS even put partial class into the framework? They removed the ability to expand/collapse all code at each scope level in C# (this was allowed in C++) because it was obviously just allowing bad habits - partial class is, IMO, the same thing. I guess my question is: Can you explain to me when there would be a legitimate reason ever to use a partial class?

EDIT: I'm aware that for Web/WinForms there is no other choice. But outside of this? Why didn't MS just put some different keyword for gluing code-genn'ed classes together? Or is there really a legit design scenario that merits it?

I do not mean this to be a rant / war thread. I'm honestly looking to learn something here. When should partial classes be used in code design? Simple question, no need to close

Thanks

Answer 1

Can you explain to me when there would be a legitimate reason to ever use a partial class?

One of the most legitimate and useful reasons is to encourage the separation of automatically generated code and your own custom extensions to it. For instance, it's common to have an automatically generated form code from some kind of designer, but you usually want to add your own specific behavior to it. This way, if you regenerate the automatic-code portion, you're not touching the part that has your specific extensions.

That said, it's quite possible to have too much of a good thing. Some tips:

• Don't make your classes partial for the sake of being partial.

• Don't put partial classes anywhere except besides one another. If you have to jump to a completely unrelated section of the project to see the other half of the class, you're probably doing it wrong.

• Don't use partial as a technique to obscure the size of the class. If you're breaking up your classes with partial because they're too big, you should revisit the Single Responsibility Principle.

• If you have three or more partial fragments for the same class, it's almost a guarantee that you're abusing partial. Two is the typical upper bound of reasonableness, and it's generally used to segment automatically-generated code from handwritten code.

Anyway, long story short - he uses partial classes, all over the place, as a way to fake multiple inheritance in C# (IMO). Why he didnt just split the classes up into multiple ones and use composition is beyond me. He will have 3 'partial class' files to make up his base class, each w/ 3-500 lines of code... And does this several times in his API.

Yes, that's definitely a clear abuse of partial!

Answer 2

There are two reasons that I would (and do) use partial classes.

1. To separate auto-generated portions of the code (such as WinForms designer code or T4 output).
2. To allow nested types their own file while still achieving the encapsulation required by your design.

Update
I can see that some are not convinced about my second point, so let me give an example; the ListViewItemCollection in the framework. It is quite rightly nested under ListView because it is only for use by ListView, but to make maintenance much easier, I would give it it's own file by using partial classes. I don't see this as bad design or a misuse of the partial keyword.

For more discussion, check out the question that this one duplicates: Partial Classes in C#