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

Answer 1

Just to add on to the previous answers that mentioned separating generated code from custom code, I've found partial classes useful for extending strongly-typed datasets.

Answer 2

Another legitimate use of partial classes is to help reduce the "monolithic web service" clutter in WCF. You want to to break it down into logical groups of functionality but don't want to have to create a ream of individual service instances/endpoints (presumably because they share state, resources, and so on).

The solution? Have the service implement multiple interfaces, and implement each interface in its own partial class. Then map different endpoints in the configuration to the same physical implementation. It makes the project a lot more maintainable, but you still only have one physical endpoint.

In some cases I'd point to this type of approach as a poor practice on account of the SRP, but when you're working with WCF services or web services in general, it's not quite so simple. You have to balance internal design requirements against external consumption requirements.

Answer 3

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

Recent versions of Visual Studio use partial classes to seperate the auto-generated designer code from your own code..

An ASP.NET example:

• Page.aspx
• Page.aspx.cs <- Your code
• Page.aspx.Designer.cs <- A partial class containing auto generated code.

A WinForms example:

• Form1.resx
• Form1.cs <- Your code
• Form1.Designer.cs <- A partial class containing auto generated code

Answer 4

Partial class exists in the .Net framework solely to let Visual Studio designers (e.g. the Asp.Net designer and the Windows Forms designer) to generate code / mess with your classes while keeping that generated code in a separate file.

(See .NET Partial Classes vs. Inheritance)

If you do something similar (generate code that needs to coexist with user-written code) then you might also find partial classes useful, but I don't believe that Microsoft ever intended partial classes as a language concept to be useful to anyone other than the Visual Studio team.

Its not so much that using Partial classes is bad design - its just you probably wont find a use for them.

Answer 5

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#

Answer 6

I fully agree with John's answer. But I would take it one step further.

• Don't make your classes partial.

The only use of partial classes I can think of that I would consider "good design" is with automatically generated code. Any other use is almost certainly unnecessarily splitting up your class. (Actually, I can see that Jeff's second point on nested classes is possibly a valid use)

Personally I think this book you are reading sounds like bad design, however do consider that he may just be using partial classes so he can just demo part of the code little bits at a time rather than just presenting the whole class in one go.