Delphi TZipFile extracts zero byte files - delphi

Question

Using Delphi XE2 and the native TZipFile I attempt to extract the contents of a downloaded zip file (which contains 2 zipped XML files) and it always extracts zero byte files.
The file is being compressed by C# code like this:
var zipFile = new ZipFile();
foreach (Tuple<string, string> t in filesMeta) {
zipFile.AddFile(string.Format("{0}{1}", StaticVariables.WebServerFileStorage, t.Item2), "").FileName = t.Item1 + ".xml";
}
response.Clear();
response.ContentType = "application/zip";
zipFile.Save(response.OutputStream);
response.End();
The Delphi extraction code is this:
zipFile := TZipFile.Create;
try
filename := 'C:\test\57f52480-ec87-4169-a820-0a65bc4ad952.zip';
if zipFile.IsValid(filename) then begin
zipFile.Open(filename, zmRead);
zipFile.ExtractAll('C:\test\');
end;
finally
zipFile.Free;
end;
I even tried using a TStream as the source instead of a file on disk. That's actually what I want to do since the zip file is downloaded from a web server into a TStream. I tried to extract the data using the overloaded open method of TZipFile to open the stream.
That got me zero byte files so I saved the zip file to disk and tried to open the file from disk and extract. Same zero byte files are extracted.
I even tried using the class method to extract the files from the zip file on disk:
System.Zip.TZipFile.ExtractZipFile(filename, 'C:\Test\');
Same zero byte files extracted.
The zip file is valid and the 2 zipped XML files can be extracted properly by both Windows 7 native file handling and 7-Zip.
Now here is something nutty...
In desperation I tried to see what the ExtractToFile() procedure
David Heffernan came up with in this question about extracting a zip to a stream would do so I tried using it like this:
var x : integer;
var fileCount : integer;
var fileNames : TArray<string>;
begin
zipFile := TZipFile.Create;
try
filename := 'C:\test\57f52480-ec87-4169-a820-0a65bc4ad952.zip';
if zipFile.IsValid(filename) then begin
zipFile.Open(filename, zmRead);
fileCount := zipFile.FileCount;
fileNames := copy(zipFile.FileNames, 0, MaxInt);
zipFile.Close;
for x := 0 to fileCount-1 do begin
// Use David Heffernan's stream procedure
ExtractToFile(filename, x, 'C:\test\' + fileNames[x]);
end;
end;
finally
zipFile.Free;
end;
end;
And David's procedure extracts the files to disk as expected! WTF???
I am really confused why a convoluted method of extraction would work and the simple extraction method would not work. I'll use David's example if I have to but I'd prefer to get the normal extract working if possible.
Any ideas are appreciated.
Cheers!
TJ

Answer 1

I had this same problem.
The source code to TZipFile shows that the TStream passed into the Read function returns the entire zip file with position set to the start of the filename you're wanting. So don't rewind. Just copyfrom or do what you want with the TStream for the uncompressed length given in the TZipHeader.
ZipStream := TStream.Create;
ZipFile.Read(MyFileName, ZipStream, ZipHeader);
//leave ZipStream pointer where it is!!!
SomethingElse.LoadFromStream(ZipStream, ZipHeader.UncompressedSize);
ZipStream.Free;
In my opinion, TZipFile should really load the ZipStream with only what is requested. The way this is implemented is not intuitive without first going through the TZipFile source code.

Answer 2

TL;DR: The solution to my problem was an external component. Zip Forge (or Abbrevia)
Read on for details.
Nothing I tried except for the roundabout way of saving the file and re-opening it using David's function worked. While that would have worked, it was not optimal as it required me to save the downloaded file to disk first and reopen it for extract and then delete the zip file. My goal was to just open the downloaded stream and extract the files directly to disk. One write to disk and no temporary file.
We even tried two different C# libraries to zip the files and both gave the same results on the streamed data. The Delphi TZipFile component could not handle it.
It turns out we have a license to ZipForge which I had forgotten about since I had not used it in ages it and that handles the download stream from the C# web server and extracts the files successfully.
For reference, I also tried the Abbrevia component version 5.2 and that also successfully extracted the files from the stream.
Hopefully this will help someone else.
All the suggestions by David and Uwe were appreciated.
Cheers!
TJ