function StringIndex(const SearchString: string; StrList: array of string): Integer;
var
  I: Integer;
begin
  Result:= -1;
  for I:= 0 to High(StrList) do
    if CompareText(SearchString, StrList[I]) = 0 then
    begin
      Result:= I;
      Break;
    end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  case StringIndex('A', ['A', 'B', 'C']) of
   -1: ShowMessage('Not in the list');
    0: ShowMessage('A');
    1: ShowMessage('B');
    2: ShowMessage('C');
  end;
end;

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