While I was reading code, I found that in the KeyDown event you can set Key := 0; to stop processing the event any further. For example: TIncrementalForm.FormKeyDown is coded as:

procedure TIncrementalForm.FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if Key = VK_RETURN then begin
    Key := 0; // Stop processing by input window
    if Shift = [ssShift] then
      btnPrevClick(nil)
    else
      btnNextClick(nil);
  end;
end;

Another example is from unit FindFrm:

procedure TFindForm.cboFindTextKeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if Key = VK_TAB then begin
    cboFindText.SelText := #9;
    Key := 0; // prevent propagation
  end;
end;

I tested it myself, but it doesn’t work:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    Memo2: TMemo;
    Button1: TButton;
    procedure Memo1KeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
  private const
    VK_X = Ord('X');
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Memo1KeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  if Key = VK_X then
  begin
    Key := 0;
    Memo2.Lines.Add('yes');
  end;
end;

end.

My intention is: whenever the user holds down the key x in the TMemo control I do some business logic (i.e. add "yes" to the memo) and stop further processing. But the result is that the key x is still inserted to the TMemo text. I want to know how to disable the default key holding down event behavior (inserting the corresponding key).

>Solution :

The key here (no pun intended) is to use OnKeyPress instead of OnKeyDown:

procedure TForm1.Memo1KeyPress(Sender: TObject; var Key: Char);
begin
  if Key = 'x' then
  begin
    Key := #0;
    Memo1.Lines.Add('yes');
  end;
end;