title: Combo Box parameter has no default value in delphi
body: <p>The program I'm working on uses an if statement to add a line to the SQL for the contents of another combo box</p>
  
  <pre><code>procedure TFmNewGarage.ComboBoxCountryEnter(Sender: TObject);
  begin
    ADOQueryCountry.SQL.Clear;
    ADOQueryCountry.SQL.Add('SELECT DISTINCT Country');
    ADOQueryCountry.SQL.Add(' FROM TblBaseCar');
    ADOQueryCountry.Open;
    while not ADOQueryCountry.Eof do
    begin
    ComboBoxCountry.Items.Add(ADOQueryCountry['Country']);
    ADOQueryCountry.Next;
    end;
  end;
  
  procedure TFmNewGarage.ComboBoxCountryChange(Sender: TObject);
  begin
    SelA:=True;
    ComboBoxManufacturer.Show;
    ComboBoxCountry.Hide;
  end;
  
  procedure TFmNewGarage.ComboBoxManufacturerEnter(Sender: TObject);
  begin
    ADOQueryManufacturer.SQL.Clear;
    ADOQueryManufacturer.SQL.Add('SELECT DISTINCT Manufacturer');
    ADOQueryManufacturer.SQL.Add(' FROM TblBaseCar');
    if SelA=true then
    ADOQueryManufacturer.SQL.Add(' WHERE Country=(ComboBoxCountry.seltext)');
    ADOQueryManufacturer.Open;
    while not ADOQueryManufacturer.Eof do
    begin
    ComboBoxManufacturer.Items.Add(ADOQueryManufacturer['Manufacturer']);
    ADOQueryManufacturer.Next;
    end;
  end;
  </code></pre>
  
  <p>At runtime this results in the error ComboBoxCountry.seltext has no default value, can anyone help me to rectify this?</p>