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;
      FileIntoString: string;
  end;

var
  Form1: TForm1;

implementation
{$R *.dfm}

procedure TForm1.OnCreate(Sender: TObject);
  function fileToString(fileName: String): String;
  var
    FileStream: TFileStream;
    Reader: TReader;
    myStringBuilder: TStringBuilder;

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

    myStringBuilder := TStringBuilder.Create;
    Reader.ReadListBegin;                       //I get 'Invalid property Value' error
                                                //in this line raised from the Reader object

    while not Reader.EndOfList do
      myStringBuilder.Append(Reader.ReadString);

    Reader.ReadListEnd;
    Result := myStringBuilder.ToString;

    Reader.Destroy;
    FileStream.Destroy;
  end;

var
  dlg: TOpenDialog;
begin

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

//CONVERT INTO STRING
if selectedFile <> '' then
  FileIntoString := fileToString(selectedFile);
end;
end.