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

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 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

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 4

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 5

Another possible use for partial classes would be to take advantage of partial methods to make methods selectively disappear using conditional compilation - this would be great for debug-mode diagnostic code or specialized unit testing scenarios.

You can declare a partial method kind of like an abstract method, then in the other partial class, when you type the keyword "partial" you can take advantage of the Intellisense to create the implementation of that method.

If you surround one part with conditional build statements, then you can easily cut off the debug-only or testing code. In the example below, in DEBUG mode, the LogSomethingDebugOnly method is called, but in the release build, it's like the method doesn't exist at all - a good way to keep diagnostic code away from the production code without a bunch of branching or multiple conditional compilation blocks.

// Main Part
public partial class Class1
{
    private partial void LogSomethingDebugOnly();

    public void SomeMethod()
    {
        LogSomethingDebugOnly();
        // do the real work
    }
}

// Debug Part - probably in a different file
public partial class Class1
{

    #if DEBUG

    private partial void LogSomethingDebugOnly()
    {
        // Do the logging or diagnostic work
    }

    #endif
}

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.

Answer 7

I find partial classes to be extremely helpful. Usually they are used to be able to extend autogenerated classes. I used them in one project with heavy unit tests. My UT classes had complex dependencies and it was not very practical to separate code across multiple classes.Of course it is better to use inheritance\composition but in some cases partial classes can be rally helpful.

Answer 8

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 9

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 10

Where I'm at we have a program that handles incoming files from clients. It's set up so that each client's code is in it's own class library project, which knows how to handle whatever format that client chooses to use.

The main code uses the libraries by defining a fairly extensive interface that a class in the library must implement (probably should be a few distinct interfaces, but it's too late to change it now). Sometimes that involves a lot more code in the same class than we'd normally think prudent. Partial classes allow us to break them up somewhat.

Answer 11

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 12

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.)

Answer 13

Generally, I consider it a code smell.

If your class is that complicated then it can probably be broken up into smaller reusable components.

Or it means that theres no inheritance hierarchy where there should be one.

For code generation scenarios it's good but I think code generation is another code smell.

Answer 14

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 15

Correction, as Matt pointed out, both sides of the partial need to be in the same assembly. my bad.

Answer 16

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 17

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 18

Sometimes you might find terribly old code at work that may make it close to impossible to refactor out into distinct elements without breaking existing code.

When you aren't given the option or the time to create a more genuine architecture, partial classes make it incredibly easy to separate logic where its needed. This allows existing code to continue using the same architecture while you gain a step closer to a more concrete architecture.

Answer 19

Anywhere you'd have used #region sections before probably makes more sense as separate files in partial classes.

I personally use partial classes for large classes where static members go in one file and instance members go in the other one.

Answer 20

EDIT: DSL Tools for Visual Studio uses partial classes.

Thus, it's a feature that many automatic generated code uses. Instead of using #region the automatic generated code goes to one file and the user code (also called custom code) goes to another and even in different directories so that the developer does not get confused with so many meaningless files.

It's good to have this choice which you can combine - but not forced to use -with inheritance

Also, it can be handy to separate the logic of some classes among several directories. Of course, for machines, it's the same, but it enhances the user readability experience.