Chalabi

public static void UzipOneFile(string zipPath, string directoryOutPath)        {
            var outDir = new DirectoryInfo(directoryOutPath);
            if (!outDir.Exists)
                outDir.Create();

            var archFile = new FileInfo(zipPath);
            if(!archFile.Exists)
                return;
            using (var f = File.Open(archFile.FullName, FileMode.Open)){
                System.IO.Compression.ZipArchive arch = new ZipArchive(f, ZipArchiveMode.Read);
                if (arch.Entries.Count > 0){
                    using (Stream sr = arch.Entries[0].Open()){
                        var ext = "";
                        try{
                            ext = new FileInfo(arch.Entries[0].Name).Extension;
                        }
                        catch (Exception){}
                        using (Stream sw = File.Create(outDir.FullName + "\\" + archFile.Name+ext)){
                            while (true){
                                int data = sr.ReadByte();
                                if(data == -1)
                                    break;
                                sw.WriteByte((byte) data);

                            }
                        }
                    }
                }
            }
        }

//...
UzipOneFile(@"D:\myarch.zip", @"D:\outFolder");