10 Delphi Interview Questions and Answers

Delphi, a robust and versatile programming language, has been a staple in software development for decades. Known for its strong support for rapid application development (RAD), Delphi is widely used in creating high-performance desktop, mobile, and enterprise applications. Its rich set of components and libraries, combined with an intuitive IDE, makes it a powerful tool for developers aiming to build scalable and maintainable software solutions.

This article aims to prepare you for Delphi-related technical interviews by providing a curated selection of questions and answers. By familiarizing yourself with these examples, you will gain a deeper understanding of Delphi’s core concepts and practical applications, enhancing your ability to tackle interview challenges with confidence.

Delphi Interview Questions and Answers

1. What is the VCL (Visual Component Library) and how does it differ from the FMX (FireMonkey) framework?

The Visual Component Library (VCL) is a framework for developing Windows applications in Delphi, offering a set of components integrated with the Windows API. It is optimized for Windows desktop applications, providing standard UI elements and more complex components like grids and charts.

FireMonkey (FMX), in contrast, is a cross-platform framework for applications on Windows, macOS, iOS, and Android. It uses GPU acceleration for advanced graphics and animations, supporting 3D graphics and custom styles.

Key differences include:

• Platform Support: VCL is Windows-only, while FMX is cross-platform.
• Rendering Engine: VCL uses Windows GDI; FMX uses GPU acceleration.
• Component Library: VCL is extensive for Windows; FMX is designed for cross-platform use.
• Performance: VCL is faster for Windows-specific applications; FMX may have overhead due to its cross-platform nature.
• Use Cases: VCL suits traditional Windows applications; FMX is for multi-platform apps with advanced graphics.

2. Describe the purpose and use of TDataSet in database applications.

TDataSet is an abstract class in Delphi for accessing and manipulating data from various sources like SQL databases or flat files. It provides a consistent interface for navigating, editing, and managing data, supporting CRUD operations and data-aware controls.

Example:

var
  MyTable: TTable;
begin
  MyTable := TTable.Create(nil);
  try
    MyTable.DatabaseName := 'DBDEMOS';
    MyTable.TableName := 'Customer';
    MyTable.Open;
    while not MyTable.Eof do
    begin
      ShowMessage(MyTable.FieldByName('CompanyName').AsString);
      MyTable.Next;
    end;
  finally
    MyTable.Free;
  end;
end;

In this example, TTable, a descendant of TDataSet, opens a table and iterates through records to display a field.

3. How do you handle exceptions in Delphi?

Exception handling in Delphi uses try, except, and finally blocks to manage runtime errors. The try block contains code that might raise an exception, the except block handles it, and the finally block executes code regardless of exceptions.

Example:

try
  SomeProcedure;
except
  on E: Exception do
    ShowMessage('An error occurred: ' + E.Message);
finally
  CleanUp;
end;

4. What are interfaces in Delphi and how do they differ from classes?

In Delphi, an interface is a reference type defining a set of methods without implementation. Classes implementing the interface must provide these methods. Interfaces support polymorphism and decoupling, allowing flexible designs.

Key differences between interfaces and classes:

• Implementation: Interfaces define methods; classes implement them.
• Multiple Inheritance: Interfaces support multiple inheritance; classes do not.
• Reference Counting: Interfaces use reference counting; classes require explicit memory management.
• Contract vs. Implementation: Interfaces define a contract; classes provide implementation.

Example:

type
  IMyInterface = interface
    ['{12345678-1234-1234-1234-123456789012}']
    procedure DoSomething;
  end;

  TMyClass = class(TInterfacedObject, IMyInterface)
  public
    procedure DoSomething;
  end;

procedure TMyClass.DoSomething;
begin
  WriteLn('Doing something...');
end;

var
  MyObject: IMyInterface;
begin
  MyObject := TMyClass.Create;
  MyObject.DoSomething;
end.

5. How would you implement a custom component in Delphi?

Creating a custom component in Delphi involves subclassing an existing component, customizing its behavior, and registering it for use in the IDE.

Example of a custom button component:

unit CustomButton;

interface

uses
  System.SysUtils, System.Classes, Vcl.Controls, Vcl.StdCtrls;

type
  TCustomButton = class(TButton)
  private
    FCustomProperty: String;
  protected
    procedure Click; override;
  published
    property CustomProperty: String read FCustomProperty write FCustomProperty;
  end;

procedure Register;

implementation

procedure TCustomButton.Click;
begin
  inherited;
  ShowMessage('Custom Button Clicked!');
end;

procedure Register;
begin
  RegisterComponents('Samples', [TCustomButton]);
end;

end.

In this example, TCustomButton inherits from TButton, adds a property, and overrides the Click method.

6. How do you manage memory in Delphi, particularly with regard to dynamic arrays and objects?

Memory management in Delphi involves both automatic and manual techniques. Dynamic arrays are automatically managed, with memory allocated and freed as needed. Objects require explicit management, using Create and Free methods to avoid memory leaks.

Example:

var
  MyArray: array of Integer;
  MyObject: TObject;
begin
  SetLength(MyArray, 10);
  SetLength(MyArray, 0);

  MyObject := TObject.Create;
  try
    // Use MyObject
  finally
    MyObject.Free;
  end;
end;

7. Describe the process of creating and using threads in Delphi.

In Delphi, threads are created using the TThread class. To create a thread, subclass TThread and override its Execute method with the code the thread will run.

Example:

type
  TMyThread = class(TThread)
  protected
    procedure Execute; override;
  end;

procedure TMyThread.Execute;
begin
  while not Terminated do
  begin
    Sleep(1000);
  end;
end;

var
  MyThread: TMyThread;
begin
  MyThread := TMyThread.Create(False);
end;

In this example, TMyThread is a subclass of TThread, with the Execute method containing the thread’s code.

8. How do you handle cross-platform development in Delphi using FireMonkey?

FireMonkey (FMX) is a cross-platform GUI framework in Delphi for creating applications on multiple platforms with a single codebase. It abstracts platform-specific details, allowing developers to focus on core functionality.

To handle cross-platform development with FireMonkey:

• Create a New Multi-Device Application: Start with a multi-device application project.
• Design the User Interface: Use the FireMonkey designer for a responsive UI.
• Write Platform-Agnostic Code: Use FMX components and libraries for common tasks.
• Handle Platform-Specific Code: Use conditional compilation and platform services.
• Test on Multiple Platforms: Regularly test on all target platforms.

9. Explain Delphi’s event-driven programming model and how you handle events.

Delphi’s event-driven programming model centers on events and event handlers. Components have properties linked to event handlers, which define actions when events occur.

Example:

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage('Button clicked!');
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Button1.OnClick := Button1Click;
end;

In this example, the Button1Click method is assigned to the OnClick event of Button1, executing when the button is clicked.

10. Discuss the component-based architecture of Delphi and how you create and manage components.

Delphi’s component-based architecture simplifies application development. Components encapsulate data and behavior, aiding design and maintenance. They can be visual, like buttons, or non-visual, like timers.

To create a component, use existing VCL components or create custom ones by extending existing classes. Manage components using the Object Inspector to set properties and handle events, streamlining development.