How to read an unknown type file of unknown size

I need to read the content of an unknown type and size file and temporary save it (in some kind of variable) so I use it later for transferring through serial port. As far as I understand, TFileStream is the right approach.

I did try implementing the following tutorial from http://docwiki.embarcadero.com/CodeExamples/Tokyo/en/TReader_(Delphi)

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Utils;

type
  TForm1 = class(TForm)
    procedure OnCreate(Sender: TObject);

    private
      selectedFile: string;
  end;

var
  Form1: TForm1;

implementation
{$R *.dfm}

procedure TForm1.OnCreate(Sender: TObject);
  function ReadFileContent(fileName: String): String;
  var
    FileStream: TFileStream;
    Reader: TReader;
    tByte :byte;

  begin

    FileStream := TFileStream.Create(fileName, fmOpenRead);
    Reader := TReader.Create(FileStream, $FF);

    Reader.ReadListBegin;           //I get 'Invalid property Value' error
                                    //in this line raised from the Reader object

    while not Reader.EndOfList do
    begin
      Reader.ReadVar(tByte, 1);
    end;

    Reader.ReadListEnd;

    Reader.Destroy;
    FileStream.Destroy;
  end;

var
  dlg: TOpenDialog;
begin
  selectedFile := '';
  dlg := TOpenDialog.Create(nil);
  try
    dlg.InitialDir := '.\';
    dlg.Filter := 'All files (*.*)|*.*';
    if dlg.Execute(Handle) then
      selectedFile := dlg.FileName;
  finally
    dlg.Free;
end;

if selectedFile <> '' then
  ReadFileContent(selectedFile);
end;
end.

Is there anything else I need to set for the Reader object to work properly or I should use a different approach?

I need to read the content of an unknown type and size file and save it into a string.

Since you want to save it in a string, either

1. the file is a text file, or
2. you are doing it wrong (a string can only store textual data).

Assuming the first option, you can simply do

MyStringVariable := TFile.ReadAllText('C:\myfile.txt');

(uses IOUtils).

There is also an overload of ReadAllText that you can use to specify the encoding (e.g., UTF-8 or UTF-16LE).

Update. The question was edited and now reads

I need to read the content of an unknown type and size file and save it.

Do you simply want to copy a file? If so, you can use any of the file-copying methods that are available, such as the CopyFile Win32 function, TFile.Copy from IOUtils, and many more.

Or do you want to obtain the bytes of the file so you can process it in your application? If so, my original answer is close to what you need. Just use ReadAllBytes instead of ReadAllText:

MyDynamicByteArray := TFile.ReadAllBytes('C:\logo.bmp');

where MyDynamicByteArray is a dynamic array of bytes (TArray<Byte>, that is, array of byte).