using System;

using System.IO;

using System.Text;

public class Program

{

    public static void Main()

    {

        var zipFile = new Ionic.Zip.ZipFile();

​

        var content1 = new MemoryStream(Encoding.Default.GetBytes("Content 1"));

        zipFile.UpdateEntry("test.txt", content1);

        zipFile.Save(@"C:\test.zip"); // here the Zip file is correct

        //zipFile = new Ionic.Zip.ZipFile(@"C:\test.zip"); // uncomment and it works too

        var content2 = new MemoryStream(Encoding.Default.GetBytes("Content 2"));

        zipFile.UpdateEntry("test.txt", content2);

        zipFile.Save();  // after that it is corrupt

    }

}