How can I unzip a file to a .NET memory stream?

using (ZipArchive archive = new ZipArchive(webResponse.GetResponseStream()))
{
     foreach (ZipArchiveEntry entry in archive.Entries)
     {
        Stream s = entry.Open();
        var sr = new StreamReader(s);
        var myStr = sr.ReadToEnd();
     }
} 

You can use ZipArchiveEntry.Open to get a stream.

This code assumes the zip archive has one text file.

using (FileStream fs = new FileStream(path, FileMode.Open))
using (ZipArchive zip = new ZipArchive(fs) )
{
    var entry = zip.Entries.First();

    using (StreamReader sr = new StreamReader(entry.Open()))
    {
        Console.WriteLine(sr.ReadToEnd());
    }
}

Looks like here is what you need:

using (var za = ZipFile.OpenRead(path))
{
    foreach (var entry in za.Entries)
    {
        using (var r = new StreamReader(entry.Open()))
        {
            //your code here
        }
    }
}

Zip compression support is built in:

using System.IO;
using System.IO.Compression;
// ^^^ requires a reference to System.IO.Compression.dll
static class Program
{
    const string path = ...
    static void Main()
    {
        using(var file = File.OpenRead(path))
        using(var zip = new ZipArchive(file, ZipArchiveMode.Read))
        {
            foreach(var entry in zip.Entries)
            {
                using(var stream = entry.Open())
                {
                    // do whatever we want with stream
                    // ...
                }
            }
        }
    }
}

Normally you should avoid copying it into another stream - just use it "as is", however, if you absolutely need it in a MemoryStream, you could do:

using(var ms = new MemoryStream())
{
    stream.CopyTo(ms);
    ms.Position = 0; // rewind
    // do something with ms
}