How to get the size of a compressed file without actually compressing it [SharpZipLib]

Question

In SharpZipLib we use ZipEntry like this:

            var fi = new FileInfo(file);
            var ze = new Sharp.Zip.ZipEntry(fi.Name);
            ze.DateTime = fi.LastAccessTime;
            ze.Size = fi.Length;
            stream.PutNextEntry(ze);

            var buffer = new byte[4096];
            var byteCount = 0;

            using (FileStream inputStream = File.OpenRead(fi.FullName))
            {
                byteCount = inputStream.Read(buffer, 0, buffer.Length);
                while (byteCount > 0)
                {
                    stream.Write(buffer, 0, byteCount);
                    byteCount = inputStream.Read(buffer, 0, buffer.Length);
                }
                inputStream.Close();
            }
            stream.CloseEntry();

There is a ze.CompressedSize property but it is -1 and populated only after the CloseEntry() which creates the zip file.

Is there a way to find out the Compressed size of the zip file without putting the files and actually creating it.

Are you sure it's not auto-populated after you `CloseEntry`? — bohdan_trotsenko, Dec 15 '15 at 15:04
`without putting the files` - no that's not possible. All compression algorithms work by reducing redundancy in the data, and it is not (can't be) known in advance to what extent that can be achieved. — 500 - Internal Server Error, Dec 15 '15 at 16:09

score 0 · Answer 1 · answered Dec 15 '15 at 14:32

Easiest way would be to have a dummy zip file, using ZipOutputStream, backed with an underlying MemoryStream. Write the file to that - make sure you set the same compression factor - and close the ZipOutputStream, re-open it using ZipFile, and scan the entry to obtain the compressed size.

There is an example here.

How to get the size of a compressed file without actually compressing it [SharpZipLib]

1 Answers1