Author: Tomas Rutkauskas

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:
1   
2   procedure TScratchMain.Memo1KeyPress(Sender: TObject; var Key: Char);
3   begin
4     if (Sender is TCustomEdit) and Odd(GetKeyState(VK_INSERT)) then
5       with TCustomEdit(Sender) do
6         if SelLength = 0 then
7           case Key of
8             ' '..#126, #128..#255:
9               begin
10                SelLength := 1;
11                if (SelLength > 0) and (SelText[1] = #13) then
12                  SelLength := 2;
13              end;
14          end;
15  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):
16  
17  procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
18  type
19    TSmallPoint = packed record
20      case integer of
21        0: (x, y: Smallint);
22        1: (long: integer);
23    end;
24  var
25    CaretPos: TPoint;
26    sCaretPos: TSmallPoint;
27  begin
28    if (FOverwrite) and (Edit1.SelLength = 0) then
29    begin
30      GetCaretPos(CaretPos);
31      sCaretPos.x := CaretPos.x;
32      sCaretPos.y := CaretPos.y;
33      Edit1.SelStart := SendMessage(Edit1.Handle, EM_CHARFROMPOS, 0, sCaretPos.long);
34      Edit1.SelLength := 1;
35      Edit1.SelText := Key;
36      Key := #0;
37    end;
38  end;
39  
40  procedure TForm1.Edit1KeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
41  begin
42    case Key of
43      VK_INSERT: FOverwrite := not FOverwrite;
44    end;
45  end;