Demystifying Partial Classes in Delphi

In programming, partial classes offer a powerful tool for code organization and separation. They allow developers to split the definition of a single class across multiple files, making it easier to manage large and complex codebases.

Understanding Partial Classes

In modern programming languages like C#, a partial class is a class that is split into multiple parts (files) but behaves as if it were defined in a single file. 

Each part contains a section of the class's definition, such as properties, methods, or events. 

These files are combined at compile time to create a unified class definition.

Splitting Responsibilities

One of the significant advantages of partial classes is the ability to split the implementation of a class across multiple places(files) based on responsibilities. 

For example, you can have one part dedicated to UI-related code, another for database interactions, and another for business logic. 

This separation helps keep code organized and maintainable.

Collaboration and Teamwork

Partial classes facilitate teamwork by allowing multiple developers to work on different aspects of a class simultaneously. Each developer can focus on a specific section, reducing the chances of merge conflicts and enabling parallel development.

Code Reusability:

Partial classes also promote code reusability. You can share common sections among different classes, allowing multiple classes to access shared code while keeping their own specific implementation separate.

Enhanced Maintainability:

By dividing a class into modular sections(files), partial classes make maintenance and refactoring more manageable. It is easier to locate and modify specific parts of a class without affecting other parts, reducing the risk of introducing bugs inadvertently.

Limitations:

While partial classes offer numerous benefits, it is crucial to be aware of their limitations. 

Modern languages impose certain rules for using partial classes. 

For instance, all parts of a partial class must have the same visibility (private, protected, etc.), and you cannot split properties or fields across files mostly.

Does Delphi support partial classes?

Well, not yet!

We should wait for Embarcadero to implement such a great ability in Delphi.

But!

Actually, there is something in Delphi (coming from the Pascal age) that is called include, 

using the include directive you can break or split your code somehow, it's not the same as a partial class in C# but still useful.

let me show you this ability with a piece of code:

  TMyClass = partial class
    procedure P1(AValue: string);
    procedure P2;
  end;
  
  procedure TMyClass.P1(AValue: string);
  begin
    {$I External.inc}
  end;

  procedure TMyClass.P2;
  begin
    ShowMessage('P2');
  end;  

How does the "TMyClass.P1" method work?
The answer is hidden in another file which is literally a text file and could be with any extension!
In my case, there is one line in this file without respecting the Pascal structure.

  ShowMessage(AValue);

I hope this wonderful feature will be added to Delphi soon.