How to set a TEdit or TMemo to overwrite instead of insert

Problem/Question/Abstract:

How to set a TEdit or TMemo to overwrite instead of insert

Answer:

Solve 1:

You have to fake it because the control does not natively support overtype mode. Provide overtype capability for edits and memos:

procedure TScratchMain.Memo1KeyPress(Sender: TObject; var Key: Char);
begin
  if (Sender is TCustomEdit) and Odd(GetKeyState(VK_INSERT)) then
    with TCustomEdit(Sender) do
      if SelLength = 0 then
        case Key of
          ' '..#126, #128..#255:
            begin
              SelLength := 1;
              if (SelLength > 0) and (SelText[1] = #13) then
                SelLength := 2;
            end;
        end;
end;

With this handler the control will start out in insert mode since the state of VK_INSERT is not toggled by default. Pressing it once will toggle the key and put the control in overtype mode. If you want it to start out in overtype, use "not Odd(...)" in the If statement.

Solve 2:

I managed to simulate it by doing this (you need to declare the FOverwrite: boolean somewhere in the form):

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
type
  TSmallPoint = packed record
    case integer of
      0: (x, y: Smallint);
      1: (long: integer);
  end;
var
  CaretPos: TPoint;
  sCaretPos: TSmallPoint;
begin
  if (FOverwrite) and (Edit1.SelLength = 0) then
  begin
    GetCaretPos(CaretPos);
    sCaretPos.x := CaretPos.x;
    sCaretPos.y := CaretPos.y;
    Edit1.SelStart := SendMessage(Edit1.Handle, EM_CHARFROMPOS, 0, sCaretPos.long);
    Edit1.SelLength := 1;
    Edit1.SelText := Key;
    Key := #0;
  end;
end;

procedure TForm1.Edit1KeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  case Key of
    VK_INSERT: FOverwrite := not FOverwrite;
  end;
end;