Appendix G: Lazarus Component Reference

This appendix provides a quick reference for the most important visual and non-visual components available in the Lazarus Component Library (LCL). For each component, you will find its purpose, key properties, key events, and a common usage pattern. Components are organized by their palette tab in the Lazarus IDE.

G.1 Standard Tab

These are the most frequently used components, found on the "Standard" tab of the component palette.

TButton

A standard push button that triggers an action when clicked.

Key Properties:

Key Events:

Common Usage:

procedure TForm1.btnCalculateClick(Sender: TObject);
begin
  lblResult.Caption := IntToStr(StrToInt(edtA.Text) + StrToInt(edtB.Text));
end;

TLabel

Displays static text. The user cannot edit it directly.

Key Properties:

Common Usage:

lblStatus.Caption := 'Processing... ' + IntToStr(Progress) + '%';
lblStatus.Font.Color := clRed;

TEdit

A single-line text input field.

Key Properties:

Key Events:

Common Usage:

procedure TForm1.edtSearchChange(Sender: TObject);
begin
  FilterList(edtSearch.Text);
end;

TMemo

A multi-line text editing control.

Key Properties:

Key Events:

Common Usage:

{ Load a file into the memo }
mmoLog.Lines.LoadFromFile('log.txt');

{ Append a line }
mmoLog.Lines.Add('Operation completed at ' + TimeToStr(Now));

{ Save to file }
mmoLog.Lines.SaveToFile('log.txt');

{ Clear }
mmoLog.Clear;

TComboBox

A dropdown list, optionally allowing the user to type custom text.

Key Properties:

Key Events:

Common Usage:

{ Populate at runtime }
cboCountry.Items.Clear;
cboCountry.Items.Add('United States');
cboCountry.Items.Add('Canada');
cboCountry.Items.Add('Mexico');
cboCountry.ItemIndex := 0;

{ Read the selected value }
SelectedCountry := cboCountry.Items[cboCountry.ItemIndex];

TCheckBox

A toggle control with a check mark.

Key Properties:

Key Events:

Common Usage:

procedure TForm1.chkBoldChange(Sender: TObject);
begin
  if chkBold.Checked then
    mmoEditor.Font.Style := mmoEditor.Font.Style + [fsBold]
  else
    mmoEditor.Font.Style := mmoEditor.Font.Style - [fsBold];
end;

TRadioButton

A mutually exclusive option button. Only one radio button in a group (same parent container) can be selected at a time.

Key Properties:

Key Events:

Common Usage:

if rbSmall.Checked then
  FontSize := 10
else if rbMedium.Checked then
  FontSize := 14
else if rbLarge.Checked then
  FontSize := 20;

Use a TRadioGroup or TGroupBox to create multiple independent groups of radio buttons on the same form.

TListBox

A scrollable list of text items from which the user can select one or more.

Key Properties:

Key Events:

Common Usage:

{ Add items }
lstFiles.Items.Add('report.txt');
lstFiles.Items.Add('data.csv');

{ Read selected item }
if lstFiles.ItemIndex >= 0 then
  OpenFile(lstFiles.Items[lstFiles.ItemIndex]);

{ Iterate selected items (multi-select) }
for I := 0 to lstFiles.Items.Count - 1 do
  if lstFiles.Selected[I] then
    ProcessFile(lstFiles.Items[I]);

TGroupBox

A container that visually groups related controls with a labeled border.

Key Properties:

Place radio buttons, checkboxes, or other controls inside a TGroupBox to group them visually and logically.

TRadioGroup

A group box specifically designed for radio buttons. Manages mutual exclusion automatically.

Key Properties:

Common Usage:

{ Set up at design time via Items property, or at runtime: }
rgSize.Items.Clear;
rgSize.Items.Add('Small');
rgSize.Items.Add('Medium');
rgSize.Items.Add('Large');
rgSize.ItemIndex := 1;   { Select 'Medium' }

{ Read selection }
case rgSize.ItemIndex of
  0: ApplySmall;
  1: ApplyMedium;
  2: ApplyLarge;
end;

G.2 Additional Tab

TBitBtn

A button that displays both an image (glyph) and text.

Key Properties:

Setting Kind automatically sets the Caption, Glyph, and ModalResult.

TSpeedButton

A flat, toolbar-style button. Does not receive focus. Often used in groups for toggle functionality.

Key Properties:

TImage

Displays an image (bitmap, PNG, JPEG, icon, etc.).

Key Properties:

Common Usage:

{ Load an image from file }
Image1.Picture.LoadFromFile('photo.png');

{ Load from resource }
Image1.Picture.LoadFromResourceName(HInstance, 'LOGO');

TShape

Draws a simple geometric shape (rectangle, circle, ellipse, rounded rectangle, etc.).

Key Properties:

TMaskEdit

A text input that enforces a specific format using an input mask.

Key Properties:

Mask Characters:

TStringGrid

A grid of cells that displays and optionally edits text in a spreadsheet-like layout.

Key Properties:

Key Events:

Common Usage:

{ Set up a simple spreadsheet }
StringGrid1.ColCount := 4;
StringGrid1.RowCount := 11;
StringGrid1.FixedRows := 1;
StringGrid1.FixedCols := 1;

{ Set headers }
StringGrid1.Cells[0, 0] := '#';
StringGrid1.Cells[1, 0] := 'Name';
StringGrid1.Cells[2, 0] := 'Score';
StringGrid1.Cells[3, 0] := 'Grade';

{ Populate data }
for I := 1 to 10 do
begin
  StringGrid1.Cells[0, I] := IntToStr(I);
  StringGrid1.Cells[1, I] := Students[I].Name;
  StringGrid1.Cells[2, I] := IntToStr(Students[I].Score);
  StringGrid1.Cells[3, I] := Students[I].Grade;
end;

{ Enable editing }
StringGrid1.Options := StringGrid1.Options + [goEditing];

G.3 Common Dialogs Tab

These are non-visual components that display standard operating system dialogs when their Execute method is called. Execute returns True if the user confirmed the dialog, False if they canceled.

TOpenDialog

Displays the standard "Open File" dialog.

Key Properties:

Common Usage:

if OpenDialog1.Execute then
begin
  mmoEditor.Lines.LoadFromFile(OpenDialog1.FileName);
  Caption := 'Editor - ' + ExtractFileName(OpenDialog1.FileName);
end;

TSaveDialog

Displays the standard "Save File" dialog.

Key Properties: Same as TOpenDialog, plus:

Common Usage:

SaveDialog1.DefaultExt := 'txt';
SaveDialog1.Filter := 'Text files|*.txt|All files|*.*';
if SaveDialog1.Execute then
  mmoEditor.Lines.SaveToFile(SaveDialog1.FileName);

TFontDialog

Displays the standard font selection dialog.

Key Properties:

Common Usage:

FontDialog1.Font := mmoEditor.Font;
if FontDialog1.Execute then
  mmoEditor.Font := FontDialog1.Font;

TColorDialog

Displays the standard color picker dialog.

Key Properties:

Common Usage:

ColorDialog1.Color := Panel1.Color;
if ColorDialog1.Execute then
  Panel1.Color := ColorDialog1.Color;

TSelectDirectoryDialog

Displays a directory (folder) picker. This is Lazarus-specific (Delphi uses a different approach).

Key Properties:

G.4 Data Access Tab

These components provide database connectivity. The examples use SQLite, but the SQLDB framework supports PostgreSQL, MySQL, Oracle, ODBC, and others by substituting the connection component.

TSQLite3Connection

Provides a connection to a SQLite database file.

Key Properties:

Common Usage:

SQLite3Connection1.DatabaseName := 'mydata.db';
SQLite3Connection1.Open;

You must have the SQLite library (sqlite3.dll on Windows, libsqlite3.so on Linux) available on your system.

TSQLTransaction

Manages database transactions. Required by SQLDB — every query must be associated with a transaction.

Key Properties:

Key Methods:

TSQLQuery

Executes SQL statements and holds result sets.

Key Properties:

Key Methods:

Common Usage:

{ Select query }
SQLQuery1.SQL.Text := 'SELECT id, name, email FROM customers WHERE active = :active';
SQLQuery1.Params.ParamByName('active').AsBoolean := True;
SQLQuery1.Open;

while not SQLQuery1.EOF do
begin
  WriteLn(SQLQuery1.FieldByName('name').AsString);
  SQLQuery1.Next;
end;
SQLQuery1.Close;

{ Insert query }
SQLQuery1.SQL.Text := 'INSERT INTO customers (name, email) VALUES (:name, :email)';
SQLQuery1.Params.ParamByName('name').AsString := 'Alice Smith';
SQLQuery1.Params.ParamByName('email').AsString := 'alice@example.com';
SQLQuery1.ExecSQL;
SQLTransaction1.Commit;

TDataSource

Acts as a bridge between a dataset (like TSQLQuery) and data-aware visual controls (like TDBGrid).

Key Properties:

G.5 Data Controls Tab

Data-aware controls automatically display and edit data from a TDataSource.

TDBGrid

Displays a dataset as a scrollable table with rows and columns.

Key Properties:

Key Events:

Common Usage:

{ Minimal setup: connect DataSource to the query, DBGrid to the DataSource }
DataSource1.DataSet := SQLQuery1;
DBGrid1.DataSource := DataSource1;
SQLQuery1.Open;    { Grid automatically populates }

TDBEdit

A single-line edit control bound to a database field.

Key Properties:

TDBMemo

A multi-line text control bound to a text or memo database field.

Key Properties:

TDBNavigator

A toolbar with buttons for navigating and editing a dataset (First, Prior, Next, Last, Insert, Delete, Edit, Post, Cancel, Refresh).

Key Properties:

Common Usage:

DBNavigator1.DataSource := DataSource1;
{ The navigator automatically enables/disables buttons based on the dataset state }

G.6 Common Patterns

Master-Detail Setup

Display a list of customers in one grid and their orders in another:

{ Master query }
qryCustomers.SQL.Text := 'SELECT * FROM customers';
qryCustomers.Open;

{ Detail query — filtered by the master's current record }
qryOrders.SQL.Text := 'SELECT * FROM orders WHERE customer_id = :customer_id';
qryOrders.DataSource := dsCustomers;   { Links to master }

{ The detail query re-executes automatically when the master record changes }

Creating a Form Programmatically

procedure TForm1.btnOpenClick(Sender: TObject);
var
  Dlg: TForm;
  Lbl: TLabel;
  Btn: TButton;
begin
  Dlg := TForm.CreateNew(Self);
  try
    Dlg.Caption := 'About';
    Dlg.Width := 300;
    Dlg.Height := 150;
    Dlg.Position := poMainFormCenter;

    Lbl := TLabel.Create(Dlg);
    Lbl.Parent := Dlg;
    Lbl.Caption := 'Programming with Pascal v1.0';
    Lbl.Left := 20;
    Lbl.Top := 20;

    Btn := TButton.Create(Dlg);
    Btn.Parent := Dlg;
    Btn.Caption := 'OK';
    Btn.ModalResult := mrOK;
    Btn.Left := 110;
    Btn.Top := 80;

    Dlg.ShowModal;
  finally
    Dlg.Free;
  end;
end;

Loading and Saving Settings with TIniFile

While not a visual component, TIniFile (from the IniFiles unit) is commonly used to save and restore control states:

uses
  IniFiles;

procedure SaveSettings;
var
  Ini: TIniFile;
begin
  Ini := TIniFile.Create('settings.ini');
  try
    Ini.WriteString('Window', 'Title', Form1.Caption);
    Ini.WriteInteger('Window', 'Width', Form1.Width);
    Ini.WriteInteger('Window', 'Height', Form1.Height);
    Ini.WriteBool('Options', 'WordWrap', chkWordWrap.Checked);
  finally
    Ini.Free;
  end;
end;

procedure LoadSettings;
var
  Ini: TIniFile;
begin
  Ini := TIniFile.Create('settings.ini');
  try
    Form1.Caption := Ini.ReadString('Window', 'Title', 'Untitled');
    Form1.Width := Ini.ReadInteger('Window', 'Width', 640);
    Form1.Height := Ini.ReadInteger('Window', 'Height', 480);
    chkWordWrap.Checked := Ini.ReadBool('Options', 'WordWrap', True);
  finally
    Ini.Free;
  end;
end;

G.7 Common Properties Shared by All Controls

Every visual control inherits from TControl and shares these properties:

Common Events Shared by All Controls

G.8 Naming Conventions

Lazarus uses a standard prefix convention for component names. Following these conventions makes code more readable:

This appendix covers the components you will use most frequently when building Lazarus applications. For the complete component reference, consult the Lazarus documentation at https://wiki.lazarus-ide.org/ and the LCL source code.