Partial Class in C#

 Partial class: 

A partial class is a special feature of C#. It provides a special ability to implement the functionality of a single class into multiple files and all these files are combined into a single class file when the application is compiled. A partial class is created by using a partial keyword. This keyword is also useful to split the functionality of methods, interfaces, or structure into multiple files.

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Rules for Partial Classes

• All the partial class definitions must be in the same assembly and namespace.
• All the parts must have the same accessibility like public or private, etc.
• If any part is declared abstract, sealed or base type then the whole class is declared of the same type.
• The user is also allowed to use nested partial types.

Advantages : 

• With the help of partial classes, multiple developers can work simultaneously in the same class in different files.

• With the help of a partial class concept, you can split the UI of the design code and the business logic code to read and understand the code.

• When you were working with automatically generated code, the code can be added to the class without having to recreate the source file like in Visual studio.

• You can also maintain your application in an efficient manner by compressing large classes into small ones.

Partial Method:

Partial classes or structs can contain a method that split into two separate .cs files of the partial class or struct. One of the two .cs files must contain a signature of the method, and other file can contain an optional implementation of the partial method. Both declaration and implementation of a method must have the partial keyword.

Rules for Partial Methods

• Partial methods must use the partial keyword and must return void.
• Partial methods can have in or ref but not out parameters.
• Partial methods are implicitly private methods, so cannot be virtual.
• Partial methods can be static methods.
• Partial methods can be generic.

Example:

EmployeeProps.cs
public partial class Employee
{
    public Employee() { 
        GenerateEmpId();
    }
    public int EmpId { get; set; }
    public string Name { get; set; }

    partial void GenerateEmployeeId();

}

EmployeeMethods.cs
public partial class Employee
{
    partial void GenerateEmployeeId()
    {
        this.EmpId = random();
    }
}

EmployeeMethods.cs
class Program
{
    static void Main(string[] args)
    {
        var emp = new Employee();
        Console.WriteLine(emp.EmpId); // prints genereted id

        Console.ReadLine();
    }
}