ZipArchive gives Unexpected end of data corrupted error

Move zipStream.ToArray() outside of the zipArchive using.

The reason for your problem is that the stream is buffered. There's a few ways to deal wtih it:

• You can set the stream's AutoFlush property to true.
• You can manually call .Flush() on the stream.

Or, since it's MemoryStream and you're using .ToArray(), you can simply allow the stream to be Closed/Disposed first (which we've done by moving it outside the using).

I Dispose ZipArchive And error solved

 public static byte[] GetZipFile(Dictionary<string, List<FileInformation>> allFileInformations)
    {

        MemoryStream compressedFileStream = new MemoryStream();
        //Create an archive and store the stream in memory.
        using (var zipArchive = new ZipArchive(compressedFileStream, ZipArchiveMode.Create, true))
        {
            foreach (var fInformation in allFileInformations)
            {
                var files = allFileInformations.Where(x => x.Key == fInformation.Key).SelectMany(x => x.Value).ToList();
                for (var i = 0; i < files.Count; i++)
                {
                    ZipArchiveEntry zipEntry = zipArchive.CreateEntry(fInformation.Key + "/" + files[i].FileName);

                    var caseAttachmentModel = Encoding.UTF8.GetBytes(files[i].Content);

                    //Get the stream of the attachment
                    using (var originalFileStream = new MemoryStream(caseAttachmentModel))
                    using (var zipEntryStream = zipEntry.Open())
                    {
                        //Copy the attachment stream to the zip entry stream
                        originalFileStream.CopyTo(zipEntryStream);
                    }
                }
            }
            //i added this line 
            zipArchive.Dispose();

            return compressedFileStream.ToArray();
        }
    }

public void SaveZipFile(){
        var zipFileArray = Global.GetZipFile(allFileInformations);
        var zipFile = new MemoryStream(zipFileArray);
        FileStream fs = new FileStream(path + "\\111.zip", 
        FileMode.Create,FileAccess.Write);
        zipFile.CopyTo(fs);
        zipFile.Flush();
        fs.Close();
        zipFile.Close();
}