How to create a combobox with two columns (one hidden) in Delphi 7?

Question

How to create a TComboBox with two columns that has one of its columns hidden so that it can keep an id value along with the actual item in it? And then how to get to that id value programmatically?

Answer 1

There's no need for two columns here.

You can take advantage of the fact that TComboBox.Items (like many other things in Delphi, like TStringList, TMemo.Lines, and TListBox.Items) descends from TStrings, which has both the Strings and Objects properties. Objects stores anything the size of a TObject, which is a pointer.

This means you can store your integer value by simply typecasting it to a TObject when adding it, and typecasting it back to an Integer when retrieving it.

Something like this should work:

procedure TForm1.FormCreate(Snder: TObject);
var
  i: Integer;
  sItem: String;
begin
  for i := 0 to 9 do
  begin
    sItem := Format('Item %d', [i]);
    ComboBox1.Items.AddObject(sItem, TObject(i));
  end;
end;

To retrieve the value:

procedure TForm1.ComboBox1Click(Sender: TObject);
var
  Idx: Integer;
  Value: Integer;
begin
  Idx := ComboBox1.ItemIndex;
  if Idx <> -1 then
  begin
    Value := Integer(ComboBox1.Items.Objects[Idx]);
    // Do something with value you retrieved
  end;
end;

Note that, since the Objects property is actually meant to store objects, this gives you a lot of flexibility. Here's an example (intentionally very trivial) of storing a customer's contact information in an associated object instance and displaying it in labels when an item from a listbox is selected.

unit Unit1;

interface

uses
  Windows, Messages, Variants, Classes, Graphics, Controls, 
  Forms, Dialogs, StdCtrls;

type
  TCustomer=class
  private
    FContact: string;
    FPhone: string;
  public
    constructor CreateCustomer(const AContact, APhone: string);
    property Contact: string read FContact write FContact;
    property Phone: string read FPhone write FPhone;
  end;

  TForm1 = class(TForm)
    ListBox1: TListBox;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    lblContact: TLabel;
    lblPhone: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure ListBox1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TCustomer }

constructor TCustomer.CreateCustomer(const AContact, APhone: string);
begin
  inherited Create;
  FContact := AContact;
  FPhone := APhone;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
var
  i: Integer;
begin
  for i := 0 to ListBox1.Items.Count - 1 do
    ListBox1.Items.Objects[i].Free;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  lblContact.Caption := '';
  lblPhone.Caption := '';

  // Create some customers. Of course in the real world you'd load this
  // from some persistent source instead of hard-coding them here.
  ListBox1.Items.AddObject('N Company', TCustomer.CreateCustomer('Nancy', '555-3333'));
  ListBox1.Items.AddObject('B Company', TCustomer.CreateCustomer('Brad', '555-1212'));
  ListBox1.Items.AddObject('A Company', TCustomer.CreateCustomer('Angie', '555-2345'));
end;

procedure TForm1.ListBox1Click(Sender: TObject);
var
  Cust: TCustomer;
begin
  if ListBox1.ItemIndex <> -1 then
  begin
    Cust := TCustomer(ListBox1.Items.Objects[ListBox1.ItemIndex]);
    lblContact.Caption := Cust.Contact;
    lblPhone.Caption := Cust.Phone;
  end;
end;

end.

Answer 2

ComboBox controls do not support columns, and you do not need a hidden column anyway to accomplish what you need.

The TComboBox.Items property is a TStrings descendant. It can hold both string values and associated user-defined data values together at the same time, but the user will only see the string values in the UI. Use the Items.AddObject() method to add string+ID values to the list, and then use the Items.Objects[] property to retrieve the ID values when needed.

Alternatively, you could just store your ID values in a separate array that has the same number of elements as the ComboBox and then use the ComboBox item indexes to access the array values. This is especially important if you need to store a value of -1, because that particular value is not retrievable from the Objects[] property of a TComboBox due to the way the getter method is implemented, like Rob said.