How read the content of file to a string in delphi XE

You should take encoding into account, for example:

function LoadFileToStr(const FileName: TFileName): String;
var
  FileStream : TFileStream;
  Bytes: TBytes;

begin
  Result:= '';
  FileStream:= TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
  try
    if FileStream.Size>0 then begin
      SetLength(Bytes, FileStream.Size);
      FileStream.Read(Bytes[0], FileStream.Size);
    end;
    Result:= TEncoding.ASCII.GetString(Bytes);
  finally
    FileStream.Free;
  end;
end;


//test
procedure TForm2.Button1Click(Sender: TObject);
begin
  ShowMessage(LoadFileToStr('C:\autoexec.bat'));
end;

you code does not work because you are reading the content of the file using a unicode string as buffer, so you are just moving bytes from the internal buffer of the TFileStream to the unicode string ignoring the encoding.

you can fix easily your procedure , just changing the result type to AnsiString

function LoadFileToStr(const FileName: TFileName): AnsiString;

but i will recommend you which you use the TFile.ReadAllText function instead which in a single line of code read the content of a file a also handle the encoding of the file.

I recommend using a TStringList to load the content of the file and then set the result to variable.Text, for example:

function LoadFileToStr(const FileName: TFileName): String;  
var LStrings: TStringList;  
begin  
    LStrings := TStringList.Create;
    try  
      LStrings.Loadfromfile(FileName);  
      Result := LStrings.text;  
    finally  
      FreeAndNil(LStrings);  
    end;  
end;

In this way you don't have to worry about anything, it will be backwards and future compatible IMHO.

EDIT: If you need to load from a TStream descendant, then replace LoadFromFile with LoadFromStream.

You can achieve this with one line of code using ReadAllText function. Like this:

Uses IOUtils;

TFile.ReadAllText(FileName);

or

TFile.ReadAllText(FileName, s, TEncoding.ASCII) // if you want to force ASCII (other) encoding 

It will correctly detect ANSI, Unicode and binary files.