LastKeyPressed: Word;

procedure TForm1.ComboBox1KeyDown Sender: TObject; var Key: Word;
Shift: TShiftState);
begin
LastKeyPressed := Key; // 保存上一个你按的键
end;

procedure TForm1.ComboBox1Change Sender: TObject);
Var
ComboText : String;
NewComboText : String;
matchindex : integer;
begin
ComboText := ComboBox1.Text;
// 使用户能删除文本;
if  LastKeyPressed = VK_DELETE) OR  LastKeyPressed = 8 ) then begin
LastKeyPressed := 0;
end
else begin
if ComboBox1.SelStart <> length  ComboText) then // 确定用户此时刚开始打字
// 不做任何事，因为这可以让用户在文本中间任意打字
else begin
// 寻找匹配
matchindex := sendmessage   ComboBox1.Handle, CB_FINDSTRING , -1,
LPARAM ComboText) )
if matchindex >= 0 then begin
ComboBox1.ItemIndex := matchindex
NewComboText := ComboBox1.Text;
// 选择文本的其余部分:
sendmessage   ComboBox1.Handle, CB_SETEDITSEL , 0,
MAKELPARAM length ComboText) , word -1) ));
end;
end;
end;