[SOLVED] How to set DBCombobox fields with a red border to show to user as a mandatory field?

How to set DBCombobox fields with a red border to show to user as a mandatory field?

Similar to the question which has been posted How to set required Edit fields with a red border? answered by Mr. Bummi

I would like to know, is it possible for using the same implementation for DBCombobox or not?

Solution

You could use the following (it is verified on Delphi 10 Seattle).

  TDBCombobox = Class(VCL.DBCtrls.TDBCombobox)
  private
    FBordercolor: TColor;
    procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
    procedure SetBorderColor(AColor: TColor);
  published
    property BorderColor: TColor read FBorderColor write SetBorderColor default clBlack;
  end;


    ....
    
Implementation

procedure TDBCombobox.WMPaint(var Message: TWMPaint);
begin
  inherited;
  Self.Canvas.Pen.Color := FBorderColor;
  Self.Canvas.Pen.Width := 1;
  Self.Canvas.Polyline([Point(0, 0), Point(Width-1, 0), Point(Width - 1, Height - 1),
                        Point(0, Height - 1),Point(0, 0)]);
end;

procedure TDBCombobox.SetBorderColor(AColor: TColor);
begin
  FBorderColor := AColor;
  invalidate;
end;

It is the same idea like the post you linked, You use a hack to add features to standard components.

If it is worth it, then you should create a descendant like @kobik said.

If a standard component needs a new feature, then it is not a standard any more.