Partial Classes in C#

Question

Are there are good uses of Partial Classes outside the webforms/winforms generated code scenarios? Or is this feature basically to support that?

Answer 1

I use partial classes as a means of separating out the different sub elements of custom controls that I write. Also, when used with entity creation software, it allows products like LLBLGen to create generated versions of classes, as well as a custom, user edited version, that won't get replaced if the entities need to be regenerated.

Answer 2

It is in part to support scenarios (WebForms, WinForms, LINQ-to-SQL, etc) mixing generated code with programmer code.

There are more reasons to use it. For example, if you have big classes in large, unwieldy files, but the classes have groups of logically related methods, partial classes may be an option to make your file sizes more manageable.

Answer 3

As mentioned earlier, I too think this is a code smell.

If a class is so big that it needs to be split into more files, means that it is breaking the single responsibility principle and doing too many things. The large class could be broken down into smaller classes that cooperate together.

If you have to use partial classes or regions to organize code, consider if they should be in their own classes. It increases readability and you'd get more code reuse.

Answer 4

I use it in a data access layer. The generated classes like the mapper and queries a partial. If I need to add a mapper method for example to do a fancy load that's not generated I add it to the custom class.

At the end the programmer that uses the data layer in the business layer only sees one class with all the functionality he or she needs. And if the data source changes the generic parts can easily be generated without overwriting custom stuff.

Answer 5

I often use partial classes to give each nested class its own file. There have been some architectures I've worked on where most of the implementation was only required by one class and so we nested those classes in that one class. It made sense to keep the files easier to maintain by using the partial class ability and splitting each one into its own file.

We've also used them for grouping stock overrides or the hiding of a stock set of properties. Things like that. It's a handy way of mixing in a stock change (just copy the file and change the partial class name to the target class - as long as the target class is made partial too, of course).

Answer 6

I worked on a project a couple years ago where we had a typed DataSet class that had a ton of code in it: Methods in the DataTables, methods in the TableAdapters, declarations of TableAdapter instances, you name it. It was a massive central point of the project that everyone had to work on often, and there was a lot of source-control contention over the partial class code file.

So I split the code file into fix or six partial class files, grouped by function, so that we could work on smaller pieces and not have to lock the whole file every time we had to change some little thing.

(Of course, we could also have solved the problem by not using an exclusively-locking source-control system, but that's another issue.)