Here's how to set all the important properties from code (in the form's OnCreate event handler):

procedure TForm1.FormCreate(Sender: TObject);
begin
 with DBLookupComboBox1 do
 begin
   DataSource := DataSource1; // -> AdoTable1 -> DBGrid1
   ListSource := DataSource2;
   DataField   := 'AuthorEmail'; // from AdoTable1 - displayed in the DBGrid
   KeyField  := 'Email';
   ListFields := 'Name; Email';
   
   Visible    := False;
 end;

 DataSource2.DataSet := AdoQuery1;

 AdoQuery1.Connection := AdoConnection1;
 AdoQuery1.SQL.Text := 'SELECT Name, Email FROM Authors';
 AdoQuery1.Open;
end;

Note: when you want to display more than one field in a DBLookupComboBox, as in the example above, you have to make sure that all columns are visible. This is done by setting the DropDownWidth property. However, you'll see that initially you have to set this to a very large value which results in dropped list being too wide in most cases. One workaround is to set the DisplayWidth of a particular Field shown in a drop down list. The next code snippet, placed inside the OnCreate event for the form, ensures that both author name and it's email are displayed inside the drop down list:
 AdoQuery1.FieldByName('Email').DisplayWidth:=10;
 AdoQuery1.FieldByName('Name').DisplayWidth:=10;
 AdoQuery1.DropDownWidth:=150;

Magic...

First, we need to make sure the DBLookupComboBox1 is moved (and sized) over the cell in which the AuthorEmail field is displayed.

procedure TForm1.DBGrid1DrawColumnCell
  (Sender: TObject; 
   const Rect: TRect; 
   DataCol: Integer; 
   Column: TColumn; 
   State: TGridDrawState);
begin
  if (gdFocused in State) then
  begin
    if (Column.Field.FieldName = DBLookupComboBox1.DataField) then
    with DBLookupComboBox1 do 
    begin
      Left := Rect.Left + DBGrid1.Left + 2;
      Top := Rect.Top + DBGrid1.Top + 2;
      Width := Rect.Right - Rect.Left;
      Width := Rect.Right - Rect.Left;
      Height := Rect.Bottom - Rect.Top;

      Visible := True;
    end;
  end
end;

Next, when we leave the cell, we have to hide the combo box:

procedure TForm1.DBGrid1ColExit(Sender: TObject);
begin
  if DBGrid1.SelectedField.FieldName = DBLookupComboBox1.DataField then 
    DBLookupComboBox1.Visible := False
end;

Next, note that when in editing mode, all keystrokes are going to the DBGrid's cell, we have to make sure they are sent to the DBLookupComboBox. In the case of a DBLookupComboBox we are primarily interested in the [Tab] key - [Tab] should move the input focus to the next cell.

procedure TForm1.DBGrid1KeyPress(Sender: TObject; var Key: Char);
begin
  if (key = Chr(9)) then Exit;

  if (DBGrid1.SelectedField.FieldName = DBLookupComboBox1.DataField) then
  begin
    DBLookupComboBox1.SetFocus;
    SendMessage(DBLookupComboBox1.Handle, WM_Char, word(Key), 0);
  end
end;

That's it. Run the project and voila ... one nicely looking drop down list box (with two columns!) all over the AuthorEmail field's column.
What does happen when you select a value (Name; Email) from the drop down list? When you pick an item ("row") from a DBLookupComboBox, the value or the corresponding KeyField field is stored as the value of the DataField field.

If you need any help, after you download and explore the full source code for this project, I encourage you to post any questions on the Delphi Programming Forum.

Need more DBGrid related articles?