Partial Classes In C# with Real Example

Partial Class in C#

In this article, I explain partial classes in C# language with examples. A partial class splits the definition of a class over two or more source (.cs) files. You can create a class definition in multiple physical files but it will be compiled as one class when the classes are compiled.

Suppose you have a “Person” class. That definition is divided into two physical source files, Person1.cs and Person2.cs. Then these two files have a class that is a partial class. You compile the source code and then create it in a single class. Let's see that in Figure 1. 

Partial class and compiled class

Figure 1: Partial class and compiled 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, if you are developing a web application using Visual Studio and adding 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 the user interface control definition.

  2. When working with an 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 the 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 work the code for the same 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 implementation. 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 realted 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 when you are developing a partial class in your application.

  1. You need to use a partial keyword in each part of the partial class.
  2. The name of each part of the partial class should be the same but the source file name for each part of the partial class 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 a partial 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 the partial class is abstract then the entire class will be an abstract class.


I will develop an example that explains how to use a partial class in your project. Suppose you are working with LINQ to SQL in your application. So you create a data context, in other words, a .dbml file, and drag and drop the necessary tables. Each table creates a partial class in the data context designer file and each table field becomes a property for the table. Suppose you have a “Person” table that has the three fields "Id”, “Name” and “DateOfBirth” and you want to show the age of each person in a grid view.

What will you do? If you add a new column to the table for age in a database for the “Person” table then it fails the normalization rule so you should not do that. If you add a new property to auto-generated code then it will not be mapped to the database. So you need to create a partial class portion in a separate source file that has the “Age” property. This “Age” property calculates the age of the person when you bind a person list to the grid view. Let's see each step-by-step.

  1. Create a “Person” table in the database and insert temp data into it using the below SQL statement.

    You need to create a person table in the database that has the three fields "Id”, “Name” and “DateOfBirth”. The "Id” field is the primary key.

    CREATE TABLE Person (
          Id INT IDENTITY (1, 1) PRIMARY KEY
        , Name NVARCHAR(50)

    Above SQL CREATE statement create a table in a database called “Person”. Now insert the data into this table by using below SQL statement.

    INSERT INTO Person
    VALUES ('Sandeep', '11/29/1988 12:00:00');
    INSERT INTO Person
    VALUES ('Raviendra', '08/10/2015 12:00:00');

    Above SQL INSERT statement, insert temp data into table “Person”.

  2. Create a web application from Visual Studio. If you're a new to Visual Studio and looking to learn How to install Visual Studio 2022.

  3. Right-click on the project in the Solution Explorer, then go to “Add” and click on “Class”.

  4. Choose “LINQ to SQL Classes” from the list and provide the name “Person” for the DBML name. Then click on “Add”.

  5. Drag the User table from the database in the Server Explorer and drop it onto the O/R Designer surface of the “Person.dbml” file.

    Person entity

    Figure 2: Person entity

  6. Now you can open the “Person.designer.cs” file. In the file, the “Person” partial class has been created for dragging and dropping a “Person” table from the database on the O/RM surface.

  7. Create a partial class part that has the “Age” property to calculate the age. This file is named “PersonExtension.cs”.

    using System;  
    namespace PartialClassExample  
       public partial class Person  
           public int Age  
               get { returnConvert.ToInt32(System.DateTime.UtcNow.Date.Year - _DateOfBirth.Value.Year); }  
  8. Create a UI design to show a person's details in the grid view.

    <%@Page Language="C#" AutoEventWireup="true" CodeBehind="PersonUI.aspx.cs" Inherits="PartialClassExample.PersonUI"%>  
    <!DOCTYPE html>  
    <html xmlns="">  
    <head id="Head1" runat="server">  
       <form id="form1" runat="server">  
           <asp:GridView ID="gridPerson" runat="server">  
  9. Write code for the “Page_Load” event to bind a grid view by person list in the code behind the file.

    using System;  
    using System.Linq;  
    namespace PartialClassExample  
       public partial class PersonUI : System.Web.UI.Page  
           protected void Page_Load(object sender, EventArgs e)  
               using (PersonDataContext context =new PersonDataContext())  
                   var query = from person in context.GetTable<Person>()  
                                select new  
                   var content = query.ToList();  
                    gridPerson.DataSource = content;  
  10. Run the application, you will see the Age column in the grid view that shows the age of each person. Let's see that in Figure 3.


    Figure 3: Output


In this article, I discussed what partial classes are in C#, their advantages, and how to use them in your applications. 
If you're interested in learning about Partial Method in C#, this guide will help you how to do it.

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