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Partials

Partials allows us to split up "definitions" into multiple files. When compiled, the compiler combines all parts into a single class file.

Partial classes

  • When working on large classes, allowing multiple developers to work on different parts.
  • When using auto-generated code (e.g., UI frameworks like WPF, WinForms, Blazor).
  • To organize related methods in separate files while keeping them under the same class.

Person.cs: 

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public partial class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public void PrintFullName()
    {
        Console.WriteLine($"{FirstName} {LastName}");
    }
}

Person_2.cs: 

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public partial class Person
{
    public int Age { get; set; }

    public void PrintAge()
    {
        Console.WriteLine($"Age: {Age}");
    }
}

Partial methods

A partial method is a method that is declared (much like forward declarations in C++) in one part of a partial class and optionally implemented in another part. If no implementation is provided, the compiler removes the declaration.

  • Must be in a partial class
  • Must return void (cannot return a value)
  • Cannot have access modifiers (public, private, etc.)
  • Cannot be virtual, abstract, or override
  • Must be partial in both declaration and implementation

User_Declaration.cs: 

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public partial class User
{
    public string Name { get; set; }

    partial void OnNameChanged(); // Partial method declaration

    public void ChangeName(string newName)
    {
        Name = newName;
        OnNameChanged(); // Calls the method if implemented
    }
}

User_Definition.cs: 

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public partial class User
{
    partial void OnNameChanged()
    {
        Console.WriteLine($"Name changed to: {Name}");
    }
}