Question

I would like to send a BackSpace control char to a TMemo like the user would actually press the BackSpace button.

My Memo is readonly and if I click a button it should delete the last char in the memo.
I would like to do that without using Memo.Text := … (so no redraw or beginupdate, etc.)
Is that possible, if yes, how?

Thank you for your help.

EDIT: I tried to add #8 but no luck…

Answer 1

Sending a key press to a read only memo won’t work. The key press will be ignored because the memo is read only. To delete the final character of a memo in an efficient way, that is without replacing the entire contents, you can use EM_SETSEL and EM_REPLACESEL.

var
  Len: Integer;
begin
  Len := Memo1.GetTextLen;
  SendMessage(Memo1.Handle, EM_SETSEL, Len-1, Len);
  SendMessage(Memo1.Handle, EM_REPLACESEL, 0, LPARAM(PChar('')));
end;

Or if you prefer a pure VCL version which wraps up these Windows messages:

begin
  Memo1.SelStart := Memo1.GetTextLen-1;
  Memo1.SelLength := 1;
  Memo1.SelText := '';
end;

The latter probably sends a few more Windows messages, but is much easier to read. I would prefer the latter option.

One possible problem I can see with this is that it may not do what you want with line breaks. Since a Windows line break is two characters (CR+LF), you would need to delete two characters if the last character in the memo was LF. To handle that you can probably do it like this:

begin
  Memo1.SelStart := Memo1.GetTextLen-1;
  Memo1.SelLength := 1;
  if Memo1.SelText=#10 then
  begin
    Memo1.SelStart := Memo1.SelStart-1;
    Memo1.SelLength := 2;
  end;
  Memo1.SelText := '';
end;