This snippet will list all the filenames of a zip archive. The filenames are relative to the zip root.

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Read))
{
    for (int i = 0; i < archive.Entries.Count; i++)
    {
        Console.WriteLine($"{i}: {archive.Entries[i]}");
    }
}

Extracting all the files into a directory is very easy:

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Read))
{
    archive.ExtractToDirectory(AppDomain.CurrentDomain.BaseDirectory);
}

When the file already exists, a System.IO.IOException will be thrown.

Extracting specific files:

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Read))
{
    // Get a root entry file
    archive.GetEntry("test.txt").ExtractToFile("test_extracted_getentries.txt", true);

    // Enter a path if you want to extract files from a subdirectory
    archive.GetEntry("sub/subtest.txt").ExtractToFile("test_sub.txt", true);

    // You can also use the Entries property to find files
    archive.Entries.FirstOrDefault(f => f.Name == "test.txt")?.ExtractToFile("test_extracted_linq.txt", true);

    // This will throw a System.ArgumentNullException because the file cannot be found
    archive.GetEntry("nonexistingfile.txt").ExtractToFile("fail.txt", true);
}

Any of these methods will produce the same result.

To update a ZIP file, the file has to be opened with ZipArchiveMode.Update instead.

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Update))
{
    // Add file to root
    archive.CreateEntryFromFile("test.txt", "test.txt");

    // Add file to subfolder
    archive.CreateEntryFromFile("test.txt", "symbols/test.txt");
}

There is also the option to write directly to a file within the archive:

var entry = archive.CreateEntry("createentry.txt");
using(var writer = new StreamWriter(entry.Open()))
{
    writer.WriteLine("Test line");
}