function RandomString(ACharSet: array of Char; ALength: Integer): string;
var
  I: Integer;
begin
  Randomize;
  SetLength(Result, ALength);
  for I := 1 to ALength do
    Result[I] := ACharSet[Random(Length(ACharset) - 1)];
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  I: Integer;
const
  CharSet: array[0..10] of Char = ('A', 'a', 'B', 'b', 'C', 'c', 'D', 'd', 'E',
    'e', '0');
begin
  ShowMessage(RandomString(CharSet, 50));
end;