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 am late in the game... but just my 2 cents...

One use could be to refactor an existing god class in an existing legacy code base to multiple partial classes. It could improve the discoverability of code - if proper naming convention is being followed for the file names containing the partial classes. This could also reduce the source code repository - resolve and merge to an extent.

Ideally, a god class should be broken down into multiple small classes - each having single responsibility. Sometimes it is disruptive to perform medium to large refactorings. In such cases partial classes could provide a temporary relief.

Answer 2

Code generation was the driving force behind partial classes. The need comes from having a code-generated class that is constantly changing, but allow developers to supply custom code as part of the class that will not be overridden everytime changes are made that force the class to be regenerated.

Take WinForms or Typed-DataSets for example (or any designer for that matter). Everytime you make a change to the designer it serializes the corresponding code to a file. Let's say you need to provide a few additional methods that the generator doesn't know anything about. If you added it to the generated file your changes would be lost the next time it was generated.

A project that I'm currently working on uses code-generation for all the DAL, BLL, and business entities. However, the generator only get's us 75% of the information. The remaining portion has to be hand coded (custom business logic for instance). I can assume that every BLL class has a SelectAll method, so that's easy to generate. However My customer BLL also needs to have a SelectAllByLocation method. I can't put this in my generator because it's not generic to all BLL classes. Therefore I generate all of my classes as partial classes, and then in a separate file I define my custom methods. Now down the road when my structure changes, or I need to regenerate my BLL for some reason, my custom code won't get wiped out.

Answer 3

On UserControls which are relatively complicated, I put the event handling stuff in one file and the painting and properties in another file. Partial classes work great for this, Usually these parts of the class are relatively independent and it's nice to be able to edit painting and event handling side by side.

Answer 4

maybe its too late but please let me to add my 2 cents too:

*.When working on large projects, spreading a class over separate files allows multiple programmers to work on it simultaneously.

*.You can easily write your code (for extended functionality) for a VS.NET generated class. This will allow you to write the code of your own need without messing with the system generated code

Answer 5

I just found a use for partial classes. I have a [DataContract] class that I use to pass data to the client. I wanted the client to be able to display the class in a specific way (text output). so I created a partial class and overrode the ToString method.

Answer 6

LINQ to SQL makes good use of partial classes to extend designer generated code. I think you will typically find this pattern of partial classes being used by designer-created code.