Partial Classes in C#

partial class splits the definition of a class over two or more source files. You can create a class definition in multiple files but it will be compiled as one class.

Advantages of a Partial Class

Here is a list of some of the advantages of partial classes:

  1. You can separate UI design code and business logic code so that it is easy to read and understand. For example, you are developing a 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.
  2. 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.
  3. More than one developer can simultaneously write the code for the class.
  4. 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. Let’s see the following code snippet.
    public interface IRegister
    {
        //Register related function
    }
    
    public interface ILogin
    {
        //Login related function
    }
    
    //UserRegister.cs file
    public partial classUser : IRegister, ILogin
    {
        //implements IRegister interface
    } 
    
    //UserLogin.cs file
    public partial classUser
    {
        //implements ILogin interface
    }

Points That You Should be Careful about Partial Classes

There are some points that you should be careful about when you are developing a partial class in your application.

  1. You need to use partial keyword in each part of partial class.
  2. The name of each part of partial class should be the same but source file name for each part of partialclass can be different.
  3. All parts of a partial class should be in the same namespace.
  4. Each part of a partial class should be in the same assembly or DLL, in other words you can’t create apartial class in source files of a different class library project.
  5. Each part of a partial class has the same accessibility.
  6. If you inherit a class or interface on a partial class, then it is inherited on all parts of a partial class.
  7. If a part of a partial class is sealed, then the entire class will be sealed.
  8. If a part of partial class is abstract, then the entire class will be an abstract class.
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By Sriramjithendra Posted in C#.NET

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