/*
The following example shows how to complete partial strings
typed into a combo box. The code represents the OnKeyPress
event handler of the combo box, which performs most of the
default keystroke handling before finding a matching list
item and updating the text.
Note:   This OnKeyPress event handler does not deal with the
case when the user types the Delete key. That case must be
caught in the OnKeyDown event handler instead.
*/
void __fastcall TForm1::ComboBox1KeyPress(TObject *Sender, char &Key)
{
  // first, process the keystroke to obtain the current string
  // This code requires all items in list to be uppercase
  if (Key >= 'a' && Key <= 'z')
    Key -= 32; // Force Uppercase only!
  TComboBox *pCB = dynamic_cast<TComboBox *>(Sender);
  AnsiString TmpStr;
  bool BackSpace = (Key == (char)VK_BACK);
  if (BackSpace && pCB->SelLength)
    TmpStr = pCB->Text.SubString(1,pCB->SelStart)+
             pCB->Text.SubString(pCB->SelLength+pCB->SelStart+1,255);
  else if (BackSpace) // SelLength == 0
    TmpStr = pCB->Text.SubString(1,pCB->SelStart-1)+
             pCB->Text.SubString(pCB->SelStart+1,255);
  else //Key is a visible character
    TmpStr = pCB->Text.SubString(1,pCB->SelStart)+ Key +
             pCB->Text.SubString(pCB->SelLength+pCB->SelStart+1,255);
  if (TmpStr.IsEmpty())
    return;
  // set SelSt to the current insertion point
  int SelSt = pCB->SelStart;
  if (BackSpace && SelSt > 0)
    SelSt--;
  else if (!BackSpace)
    SelSt++;
  Key = 0; // indicate that key was handled
  if (SelSt == 0)
  {
    pCB->Text = "";
    return;
  }
  // Now that TmpStr is the currently typed string, see if we can locate a match
  bool Found = false;
  for (int i = 1; i < pCB->Items->Count; i++)
    if (TmpStr == pCB->Items->Strings[i-1].SubString(1,TmpStr.Length()))
    {
      pCB->Text = pCB->Items->Strings[i-1]; // update to the match that was found
      pCB->ItemIndex = i-1;
      Found = true;
      break;
    }
  if (Found) // select the untyped end of the string
  {
    pCB->SelStart = SelSt;
    pCB->SelLength = pCB->Text.Length() - SelSt;
  }
  else Beep();
}

TStringList *CharSetList;

void __fastcall TForm1::AddCharacterSet(String S)
{
  CharSetList->Add(S);
}

#include <memory>       //for STL auto_ptr class

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  static std::auto_ptr<TStringList> _CharSetListCleaner(CharSetList = new TStringList);
  GetCharsetValues(AddCharacterSet);
  CharSetList->Sort();
  ComboBox1->Items = CharSetList;
}

{
The following example shows how to complete partial strings
typed into a combo box. The code represents the OnKeyPress
event handler of the combo box, which performs most of the
default keystroke handling before finding a matching list
item and updating the text.
Note:   This OnKeyPress event handler does not deal with the
case when the user types the Delete key. That case must be
caught in the OnKeyDown event handler instead.
Note: Set the TComboBox AutoComplete to false to turn off
default auto completion.
}
procedure TForm1.ComboBox1KeyPress(Sender: TObject; var Key: Char);
var
  Found: boolean;
  i, ct, SelSt: Integer;
  TmpStr: string;
begin
  { first, process the keystroke to obtain the current string }
  { This code requires all items in list to be uppercase}
  if Key in ['a'..'z'] then Dec(Key,32); {Force Uppercase only!}
  ct := (Sender as TComboBox).Items.Count;
  with (Sender as TComboBox) do
  begin
    SelSt := SelStart;
    if (Key = Chr(vk_Back)) and (SelLength <> 0) then
     TmpStr := Copy(Text, 1, SelStart) + Copy(Text, SelLength + SelStart + 1, 255)
    else if Key = Chr(vk_Back) then {SelLength = 0}
     TmpStr := Copy(Text, 1, SelStart-1) + Copy(Text, SelStart + 1,255)
    else {Key in ['A'..'Z', etc]}
     TmpStr := Copy(Text, 1, SelStart) + Key + Copy(Text, SelLength + SelStart + 1, 255);
    if TmpStr = '' then Exit;
    { update SelSt to the current insertion point }
    if (Key = Chr(vk_Back)) and (SelSt > 0) then Dec(SelSt)
    else if Key <> Chr(vk_Back) then Inc(SelSt);
    Key := #0; { indicate that key was handled }
    if SelSt = 0 then
    begin
      Text:= '';
      Exit;
    end;
   {Now that TmpStr is the currently typed string, see if we can locate a match }
    Found := False;
    for i := 1 to ct do
    begin
      if Copy(Items[i - 1], 1, Length(TmpStr)) = TmpStr then
      begin
        Text := Items[i-1]; { update to the match that was found }
        ItemIndex := i-1;
        Found := True;
        Break;
      end;
    end;
    if Found then { select the untyped end of the string }
    begin
      SelStart := SelSt;
      SelLength := Length(Text) - SelSt;
    end
    else Beep;
  end;
end;

var
  CharSetList : TStringList;

procedure TForm1.AddCharacterSet(const S: string);
begin
  CharSetList.Add(S);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  CharSetList := TStringList.Create;
  Graphics.GetCharsetValues(AddCharacterSet);
  ComboBox1.Items := CharSetList;
end;