using System;

using System.Reflection;

using System.IO;

using System.IO.Compression;

​

public static class ZipArchiveEntryExtension {

    static readonly FieldInfo field = typeof(ZipArchiveEntry).GetField("_crc32",

                         BindingFlags.NonPublic | 

                         BindingFlags.Instance);

    public static UInt32 Crc32(this ZipArchiveEntry entry)

    {

        return (UInt32)field.GetValue(entry);

    }

}

public class Program

{

    public static void Main()

    {       

        MemoryStream s = new MemoryStream();

        ZipArchive archive = new ZipArchive(s, ZipArchiveMode.Create);

        ZipArchiveEntry entry = archive.CreateEntry("crcTestEntry");

        Console.WriteLine(entry.Crc32());

        using (var es = entry.Open()) {

            es.WriteByte(0x55); 

        }

        Console.WriteLine(entry.Crc32());

    }

}