Is there an easy way to work around a Delphi utf8-file flaw?

Question

I have discovered (the hard way) that if a file has a valid UTF-8 BOM but contains any invalid UTF8 encodings, and is read by any of the Delphi (2009+) encoding-enabled methods such as LoadFromFile, then the result is a completely empty file with no error indication. In several of my applications, I would prefer to simply lose a few bad encodings, even if I get no error report in this case either.

Debugging reveals that MultiByteToWideChar is called twice, first to get the output buffer size, then to do the conversion. But TEncoding.UTF8 contains a private FMBToWCharFlags value for these calls, and this is initialized with a MB_ERR_INVALID_CHARS value. So the call to get the charcount returns 0 and the loaded file is completely empty. Calling this API without the flag would 'silently drop illegal code points'.

My question is how best to weave through the nest of classes in the Encoding area to work around the fact that this is a private value (and needs to be, because it is a class var for all threads). I think I could add a custom UTF8 encoding, using the guidance in Marco Cantu's Delphi 2009 book. And it could optionally raise an exception if MultiByteToWideChar has returned an encoding error, after calling it again without the flag. But that does not solve the problem of how to get my custom encoding used instead of Tencoding.UTF8.

If I could just set this up as a default for the application at initialization, perhaps by actually modifying the class var for Tencoding.UFT8, this would probably be sufficient.

Of course, I need a solution without waiting to lodge a QC report asking for a more robust design, getting it accepted, and seeing it changed.

Any ideas would be very welcome. And can someone confirm this is still an issue for XE4, which I have not yet installed?

Answer 1

I ran into the MB_ERR_INVALID_CHARS issue when I first updated Indy to support TEncoding, and ended up implementing a custom TEncoding-derived class for UTF-8 handling to avoid specifying MB_ERR_INVALID_CHARS. I didn't think to use a class helper.

However, this issue is not just limited to UTF-8. Any decoding failure of any of the TEncoding classes will result in a blank result, not an exception being raised. Why Embarcadero chose that route, when most of the RTL/VCL uses exceptions instead, is beyond me. Not raising an exception on error caused a fair amount of issues in Indy that had to be worked around.

Answer 2

This can be done pretty simply, at least in Delphi XE5 (have not checked earlier versions). Just instantiate your own TUTF8Encoding:

procedure LoadInvalidUTF8File(const Filename: string);
var
  FEncoding: TUTF8Encoding;
begin
  FEncoding := TUTF8Encoding.Create(CP_UTF8, 0, 0); 
                      // Instead of CP_UTF8, MB_ERR_INVALID_CHARS, 0
  try
    with TStringList.Create do
    try
      LoadFromFile(Filename, FEncoding);
      // ...
    finally
      Free;
    end;
  finally
    FEncoding.Free;
  end;
end;

The only issue here is that the IsSingleByte property for the newly instantiated TUTF8Encoding is then incorrectly set to False, but this property is not currently used anywhere in the Delphi sources.