SNoMappingForUnicodeCharacter = 'No mapping for the Unicode character exists in the target multi-byte code page';

  Len := GetCharCount(Bytes, ByteIndex, ByteCount);
  if (ByteCount > 0) and (Len = 0) then
    raise EEncodingError.CreateRes(@SNoMappingForUnicodeCharacter);

var
  Source, Target: TBytes;
begin
  SetLength(Source, 2);
  PWideChar(Source)^ := 'Д';
  Target := TEncoding.Convert(TEncoding.Unicode, TEncoding.ASCII, Source);
  Writeln(Length(Target), ': "', PAnsiChar(Target)^, '"');
  Readln;
end;

1: "?"