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.

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