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Frequently Asked Questions

Limiting the amount of text entered in a TEdit

Question:

How can I limit the amount of text that is entered into a TEdit control so the text entered does not exceed the width of the TEdit control?

Answer:

The following two examples demonstrate limiting the text in a edit
control to the amount of text that fits in the client area of the
control to avoid scrolling text. The first example sets the MaxLength
property to the number of "W" characters that will fit in the control.
The "W" character is chosen, since it is most likely to be the widest
character in the font. This method will work well for fixed pitch
fonts, but will produce less than desirable effects when using a
variable width font. The second method traps the edit control's key
press method, and measures the text width of the current string in the
edit control and the width of the character that will be added. If the
width exceeds the client area of the edit control, the key is
discarded, and a MessageBeep is produced.

Examples:

procedure TForm1.FormCreate(Sender: TObject);
var
  cRect : TRect;
  bm : TBitmap;
  s : string;
begin
  Windows.GetClientRect(Edit1.Handle, cRect);
  bm := TBitmap.Create;
  bm.Width := cRect.Right;
  bm.Height := cRect.Bottom;
  bm.Canvas.Font := Edit1.Font;
  s := 'W';
  while bm.Canvas.TextWidth(s) < CRect.Right do
    s := s + 'W';
  if length(s) > 1 then begin
    Delete(s, 1, 1);
    Edit1.MaxLength := Length(s);
  end;
end;

{Alternatively}

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
var
  cRect : TRect;
  bm : TBitmap;
begin
  if ((Ord(Key) <> VK_TAB) and
      (Ord(Key) <> VK_RETURN) and
      (Ord(Key) <> VK_LEFT) and
      (Ord(Key) <> VK_BACK)) then begin
    Windows.GetClientRect(Edit1.Handle, cRect);
    bm := TBitmap.Create;
    bm.Width := cRect.Right;
    bm.Height := cRect.Bottom;
    bm.Canvas.Font := Edit1.Font;
    if bm.Canvas.TextWidth(Edit1.Text + Key) > CRect.Right then begin
      Key := #0;
      MessageBeep(-1);
    end;
    bm.Free;
  end;
end;

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