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

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
}