When is it appropriate to use C partial classes

Question

I was wondering if someone could give me an overview of why I would use them and what advantage I would gain in the process

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I know this question is really old but I would just like to add my take on partial classes.

One reason that I personally use partial classes is when I'm creating bindings for a program, especially state machines.

For example, OpenGL is a state machine, there are heaps of methods that can all be changed globally, however, in my experience binding something similar to OpenGL where there are so many methods, the class can easily exceed 10k LOC.

Partial classes will break this down for me and help me with finding methods quickly.

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Partial classes span multiple files   How can you use the partial modifier on a C  class declaration   With partial classes  you can physically separate a class into multiple files  This is often done by code generators   Example  With normal C  classes  you cannot declare a class in two separate files in the same project  But with the partial modifier  you can   This is useful if one file is commonly edited and the other is machine-generated or rarely edited   Here s an example to clarify   class Program       static void Main                 A A1            A A2              Contents of file A1 cs  C   using System   partial class A       public static void A1                 Console WriteLine  A1              Contents of file A2 cs  C   using System   partial class A       public static void A2                 Console WriteLine  A2              Output   A1 A2   Partial is required here   If you remove the partial modifier  you will get an error containing this text       The namespace   lt global namespace gt   already contains a definition for  A      Tip   To fix this  you can either use the partial keyword  or change one of the class names   How does the C  compiler deal with partial classes   If you disassemble the above program  using IL Disassembler   you will see that the files A1 cs and A2 cs are eliminated  You will find that the class A is present    Class A will contain the methods A1 and A2 in the same code block  The two classes were merged into one   Compiled result of A1 cs and A2 cs  C   internal class A          Methods     public static void A1                 Console WriteLine  A1               public static void A2                 Console WriteLine  A2              Summary   Partial classes can simplify certain C  programming situations  They are often used in Visual Studio when creating Windows Forms WPF programs  The machine-generated C  code is separate  Or You could find the whole description here

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The main use for partial classes is with generated code  If you look at the WPF  Windows Presentation Foundation  network  you define your UI with markup  XML   That markup is compiled into partial classes  You fill in code with partial classes of your own

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Here is a list of some of the advantages of partial classes   You can separate UI design code and business logic code so that it is easy to read and understand  For example you are developing an web application using Visual Studio and add a new web form then there are two source files   aspx cs  and  aspx designer cs    These two files have the same class with the partial keyword  The   aspx cs  class has the business logic code while  aspx designer cs  has user interface control definition   When working with automatically generated source  the code can be added to the class without having to recreate the source file  For example you are working with LINQ to SQL and create a DBML file  Now when you drag and drop a table it creates a partial class in designer cs and all table columns have properties in the class  You need more columns in this table to bind on the UI grid but you don t want to add a new column to the database table so you can create a separate source file for this class that has a new property for that column and it will be a partial class  So that does affect the mapping between database table and DBML entity but you can easily get an extra field  It means you can write the code on your own without messing with the system generated code   More than one developer can simultaneously write the code for the class   You can maintain your application better by compacting large classes  Suppose you have a class that has multiple interfaces so you can create multiple source files depending on interface implements  It is easy to understand and maintain an interface implemented on which the source file has a partial class

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It is true that Partial Class is used in auto code generation  one use can be maintaining a large class file which might have thousand lines of code  You never know your class might end up with 10 thousand lines and you don t want to create a new class with different name   public partial class Product          50 business logic embedded in methods and properties      public partial class Product          another 50 business logic embedded in methods and properties       finally compile with product class file    Another possible use could be that more than one developer can work on the same class as they are stored at different places  People might laugh but you never know it can be handful sometimes   Product1 cs  public partial class Product         you are writing the business logic for fast moving product     Product2 cs  public partial class Product          Another developer writing some business logic        Hope it makes sense

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keep everything as clean as possible when working with huge classes  or when working on a team  you can edit without overriding  or always commiting changes

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As an alternative to pre-compiler directives   If you use pre-compiler directives  namely  IF DEBUG  then you end up with some gnarly looking code intermingled with your actual Release code   You can create a seperate partial-class to contain this code  and either wrap the entire partial class in a directive  or omit that code-file from being sent to the compiler  effectively doing the same

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I find it disturbing that the word  cohesion  does not appear anywhere in these posts  until now   And I m also disturbed that anyone thinks enabling or encouraging huge classes and methods is somehow a good thing  If you re trying to understand and maintain a code-base  partial  sucks

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From MSDN   1 At compile time  attributes of partial-type definitions are merged  For example  consider the following declarations    SerializableAttribute  partial class Moon       ObsoleteAttribute  partial class Moon       They are equivalent to the following declarations    SerializableAttribute   ObsoleteAttribute  class Moon       The following are merged from all the partial-type definitions    XML comments interfaces generic-type parameter attributes class attributes members   2 Another thing  nested partial classes can be also partial   partial class ClassWithNestedClass       partial class NestedClass        partial class ClassWithNestedClass       partial class NestedClass

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Another use is to split the implementation of different interfaces  e g  partial class MyClass   IF3          main implementation of MyClass     partial class MyClass   IF1          implementation of IF1    partial class MyClass   IF2          implementation of IF2

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Service references are another example where partial classes are useful to separate generated code from user-created code   You can  extend  the service classes without having them overwritten when you update the service reference

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Partial classes make it possible to add functionality to a suitably-designed program merely by adding source files   For example  a file-import program could be designed so that one could add different types of known files by adding modules that handle them   For example  the main file type converter could include a small class   Partial Public Class zzFileConverterRegistrar     Event Register ByVal mainConverter as zzFileConverter      Sub registerAll ByVal mainConverter as zzFileConverter          RaiseEvent Register mainConverter      End Sub End Class  Each module that wishes to register one or more types of file converter could include something like    Partial Public Class zzFileConverterRegistrar     Private Sub RegisterGif ByVal mainConverter as zzFileConverter  Handles Me Register         mainConverter RegisterConverter  GIF   GifConverter NewFactory       End Sub End Class   Note that the main file converter class isn t  exposed --it just exposes a little stub class that add-in modules can hook to   There is a slight risk of naming conflicts  but if each add-in module s  register  routine is named according to the type of file it deals with  they probably shouldn t pose a problem   One could stick a GUID in the name of the registration subroutine if one were worried about such things   Edit Addendum To be clear  the purpose of this is to provide a means by which a variety of separate classes can let a main program or class know about them   The only thing the main file converter will do with zzFileConverterRegistrar is create one instance of it and call the registerAll method which will fire the Register event   Any module that wants to hook that event can execute arbitrary code in response to it  that s the whole idea  but there isn t anything a module could do by improperly extending the zzFileConverterRegistrar class other than define a method whose name matches that of something else   It would certainly be possible for one improperly-written extension to break another improperly-written extension  but the solution for that is for anyone who doesn t want his extension broken to simply write it properly   One could  without using partial classes  have a bit of code somewhere within the main file converter class  which looked like      RegisterConverter  GIF   GifConvertor NewFactory    RegisterConverter  BMP   BmpConvertor NewFactory    RegisterConverter  JPEG   JpegConvertor NewFactory    but adding another converter module would require going into that part of the converter code and adding the new converter to the list   Using partial methods  that is no longer necessary--all converters will get included automatically

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Partial classes are primarily introduced to help Code generators  so we  users  don t end up loosing all our work   changes to the generated classes like ASP NET s  designer cs class each time we regenerate  almost all new tools that generate code LINQ  EntityFrameworks  ASP NET use partial classes for generated code  so we can safely add or alter logic of these generated codes taking advantage of Partial classes and methods  but be very carefully before you add stuff to the generated code using Partial classes its easier if we break the build but worst if we introduce runtime errors  For more details check this http   www 4guysfromrolla com articles 071509-1 aspx

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Aside from the other answers     I ve found them helpful as a stepping-stone in refactoring god-classes   If a class has multiple responsibilities  especially if it s a very large code-file  then I find it beneficial to add 1x partial class per-responsibility as a first-pass for organizing and then refactoring the code   This helps greatly because it can help with making the code much more readable without actually effecting the executing behavior   It also can help identify when a responsibility is easy to refactor out or is tightly tangled with other aspects   However--to be clear--this is still bad code  at the end of development you still want one responsibility per-class  NOT per partial class    It s just a stepping-stone

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If you have a sufficiently large class that doesn t lend itself to effective refactoring  separating it into multiple files helps keep things organized    For instance  if you have a database for a site containing a discussion forum and a products system  and you don t want to create two different providers classes  NOT the same thing as a proxy class  just to be clear   you can create a single partial class in different files  like  MyProvider cs - core logic  MyProvider Forum cs - methods pertaining specifically to the forum  MyProvider Product cs - methods for products  It s just another way to keep things organized    Also  as others have said  it s about the only way to add methods to a generated class without running the risk of having your additions destroyed the next time the class is regenerated  This comes in handy with template-generated  T4  code  ORMs  etc

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Another use i saw is   Extending  a big abstract class regarding data access logic    i have various files with names Post cs Comment cs Pages cs     in Post cs   public partial class XMLDAO  BigAbstractClass      CRUD methods of post       in Comment cs   public partial class XMLDAO  BigAbstractClass      CRUD methods of comment      in Pages cs   public partial class XMLDAO  BigAbstractClass      CRUD methods of Pages

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Most people remark that partial should only be used for a class that has a generated code file or for interfaces   I disagree  and here is why   For one example  let s look at the C  System Math class    that s class   I would not attempt to stuff 70  methods all into the same single code file   It would be a nightmare to maintain   Placing each math method into individual partial class files  and all code files into a Math folder in the project  would be significantly cleaner organization   The same could would hold true for many other classes that have a large amount of diverse functionality   For example a class for managing the PrivateProfile API might benefit by being split into a clean set of partial class files in a single project folder   Personally  I also split what most people call  helper  or  utility  classes into individual partial files for each method or method functional group   For example on one project the string helper class has almost 50 methods   That would be a long unwieldy code file even using regions   It is significantly easier to maintain using individual partial class files for each method   I would just be careful using partial classes and keep all code file layout consistent throughout the project when doing this  Such as placing any class public enums and class private members into a Common cs or similarly named file in the folder  instead of spreading them out across the files unless they are specific to only the partial file they are contained in   Keep in mind that when you split a class into separate files you also lose the ability to use the text editor splitter bar that lets you view two different sections of a current file simultaneously

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One great use is separating generated code from hand-written code that belong in the same class   For example since LINQ to SQL uses partial classes you can write your own implementation of certain pieces of functionality  like Many-to-Many relationships  and those pieces of custom code won t get overwritten when you re-generate the code   The same goes for WinForms code  All the Designer generated code goes in one file that you generally don t touch  Your hand-written code goes in another file  That way  when you change something in Designer  your changes don t get blown away

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Partial classes recently helped with source control where multiple developers were adding to one file where new methods were added into the same part of the file  automated by Resharper      These pushes to git caused merge conflicts   I found no way to tell the merge tool to take the  new methods as a complete code block   Partial classes in this respect allows for developers to stick to a version of their file  and we can merge them back in later by hand   example -   MainClass cs - holds fields  constructor  etc MainClass1 cs - a developers new code as they implement MainClass2 cs - is another developers class for their new code

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I note two usages which I couldn t find explicitly in the answers   Grouping Class Items  Some developers use comments to separate different  parts  of their class  For example  a team might use the following convention   public class MyClass        Member variables     Constructors     Properties     Methods     With partial classes  we can go a step further  and split the sections into separate files  As a convention  a team might suffix each file with the section corresponding to it  So in the above we would have something like  MyClassMembers cs  MyClassConstructors cs  MyClassProperties cs  MyClassMethods cs   As other answers alluded to  whether or not it s worth splitting the class up probably depends on how big the class is in this case  If it s small  it s probably easier to have everything in one master class  But if any of those sections get too big  its content can be moved to a separate partial class  in order to keep the master class neat  A convention in that case might be to leave a comment in saying something like  See partial class  after the section heading e g      Methods - See partial class   Managing Scope of Using statements   Namespace  This is probably a rare occurrence  but there might be a namespace collision between two functions from libraries that you want to use  In a single class  you could at most use a using clause for one of these  For the other you d need a fully qualified name or an alias  With partial classes  since each namespace  amp  using statements list is different  one could separate the two sets of functions into two separate files

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The biggest use of partial classes is to make life easier for code generators   designers  Partial classes allow the generator to simply emit the code they need to emit and they do not have to deal with user edits to the file   Users are likewise free to annotate the class with new members by having a second partial class  This provides a very clean framework for separation of concerns   A better way to look at it is to see how designers functioned before partial classes   The WinForms designer would spit out all of the code inside of a region with strongly worded comments about not modifying the code  It had to insert all sorts of heuristics to find the generated code for later processing   Now it can simply open the designer cs file and have a high degree of confidence that it contains only code relevant to the designer

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Multiple Developer Using Partial Classes multiple developer can work on the same class easily   Code Generator Partial classes are mainly used by code generator to keep different concerns separate  Partial Methods Using Partial Classes you can also define Partial methods as well where a developer can simply define the method and the other developer can implement that   Partial Method Declaration only Even the code get compiled with method declaration only and if the implementation of the method isn t present compiler can safely remove that piece of code and no compile time error will occur   To verify point 4  Just create a winform project and include this line after the Form1 Constructor and try to compile the code  partial void Ontest string s      Here are some points to consider while implementing partial classes -   Use partial keyword in each part of partial class  The name of each part of partial class should be the same but the source file name for each part of partial class can be different  All parts of a partial class should be in the same namespace  Each part of a partial class should be in the same assembly or DLL  in other words you can t create a partial class in source files from a different class library project  Each part of a partial class must have the same accessibility   i e  private  public or protected  If you inherit a class or interface on a partial class then it is inherited by all parts of that partial class  If a part of a partial class is sealed then the entire class will be sealed  If a part of partial class is abstract then the entire class will be considered an abstract class

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Whenever I have a class that contains a nested class that is of any significant size complexity  I mark the class as partial and put the nested class in a separate file  I name the file containing the nested class using the rule   class name   nested class name  cs   The following MSDN blog explains using partial classes with nested classes for maintainability  http   blogs msdn com b marcelolr archive 2009 04 13 using-partial-classes-with-nested-classes-for-maintainability aspx

[c#] When is it appropriate to use C# partial classes?

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