How to zip multiple files using only .net api in c#

With the release of the .NET Framework 4.5 this is actually a lot easier now with the updates to System.IO.Compression which adds the ZipFile class. There is a good walk-through on codeguru; however, the basics are in line with the following example:

using System.Collections.Generic;
using System.IO;
using System.IO.Compression;
using System.IO.Compression.FileSystem;

namespace ZipFileCreator
{
    public static class ZipFileCreator
    {
        /// <summary>
        /// Create a ZIP file of the files provided.
        /// </summary>
        /// <param name="fileName">The full path and name to store the ZIP file at.</param>
        /// <param name="files">The list of files to be added.</param>
        public static void CreateZipFile(string fileName, IEnumerable<string> files)
        {
            // Create and open a new ZIP file
            var zip = ZipFile.Open(fileName, ZipArchiveMode.Create);
            foreach (var file in files)
            {
                // Add the entry for each file
                zip.CreateEntryFromFile(file, Path.GetFileName(file), CompressionLevel.Optimal);
            }
            // Dispose of the object when we are done
            zip.Dispose();
        }
    }
}

Use System.IO.Packaging in .NET 3.0+.

See this introduction to System.IO.Packaging

If you're able to take a .NET 4.5 dependency, there's a System.IO.Compression.ZipArchive in that universe; see walkthrough article here (via InfoQ news summary article here)