This program reproduces the error that you report:
{$APPTYPE CONSOLE}
uses
System.SysUtils, System.IOUtils;
var
FileName: string;
begin
try
FileName := TPath.GetTempFileName;
TFile.WriteAllText(FileName, 'é', TEncoding.ANSI);
TFile.AppendAllText(FileName, 'é');
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.
Here I have written the original file as ANSI. And then called AppendAllText which will try to write as UTF-8. What happens is that we end up in this function:
class procedure TFile.AppendAllText(const Path, Contents: string);
var
LFileStream: TFileStream;
LFileEncoding: TEncoding; // encoding of the file
Buff: TBytes;
Preamble: TBytes;
UTFStr: TBytes;
UTF8Str: TBytes;
begin
CheckAppendAllTextParameters(Path, nil, False);
LFileStream := nil;
try
try
LFileStream := DoCreateOpenFile(Path);
// detect the file encoding
LFileEncoding := GetEncoding(LFileStream);
// file is written is ASCII (default ANSI code page)
if LFileEncoding = TEncoding.ANSI then
begin
// Contents can be represented as ASCII;
// append the contents in ASCII
UTFStr := TEncoding.ANSI.GetBytes(Contents);
UTF8Str := TEncoding.UTF8.GetBytes(Contents);
if TEncoding.UTF8.GetString(UTFStr) = TEncoding.UTF8.GetString(UTF8Str) then
begin
LFileStream.Seek(0, TSeekOrigin.soEnd);
Buff := TEncoding.ANSI.GetBytes(Contents);
end
// Contents can be represented only in UTF-8;
// convert file and Contents encodings to UTF-8
else
begin
// convert file contents to UTF-8
LFileStream.Seek(0, TSeekOrigin.soBeginning);
SetLength(Buff, LFileStream.Size);
LFileStream.ReadBuffer(Buff, Length(Buff));
Buff := TEncoding.Convert(LFileEncoding, TEncoding.UTF8, Buff);
// prepare the stream to rewrite the converted file contents
LFileStream.Size := Length(Buff);
LFileStream.Seek(0, TSeekOrigin.soBeginning);
Preamble := TEncoding.UTF8.GetPreamble;
LFileStream.WriteBuffer(Preamble, Length(Preamble));
LFileStream.WriteBuffer(Buff, Length(Buff));
// convert Contents in UTF-8
Buff := TEncoding.UTF8.GetBytes(Contents);
end;
end
// file is written either in UTF-8 or Unicode (BE or LE);
// append Contents encoded in UTF-8 to the file
else
begin
LFileStream.Seek(0, TSeekOrigin.soEnd);
Buff := TEncoding.UTF8.GetBytes(Contents);
end;
// write Contents to the stream
LFileStream.WriteBuffer(Buff, Length(Buff));
except
on E: EFileStreamError do
raise EInOutError.Create(E.Message);
end;
finally
LFileStream.Free;
end;
end;
The error stems from this line:
if TEncoding.UTF8.GetString(UTFStr) = TEncoding.UTF8.GetString(UTF8Str) then
The problem is that UTFStr is not in fact valid UTF-8. And hence TEncoding.UTF8.GetString(UTFStr) throws an exception.
This is a defect in TFile.AppendAllBytes. Given that it knows perfectly well that UTFStr is ANSI encoded, it makes no sense at all for it to call TEncoding.UTF8.GetString.
You should submit a bug report to Embarcadero for this defect which still exists in Delphi 10 Seattle. In the meantime you should not use TFile.AppendAllBytes.
