create zip file in .net with password

Take a look at DotNetZip (@AFract supplied a new link to GitHub in the comments)

It has got pretty geat documentation and it also allow you to load the dll at runtime as an embeded file.

Unfortunately there is no such functionality in the framework. There is a way to make ZIP files, but without password. If you want to create password protected ZIP files in C#, I'd recommend SevenZipSharp. It's basically a managed wrapper for 7-Zip.

SevenZipBase.SetLibraryPath(Path.Combine(
        Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) ?? Environment.CurrentDirectory,
        "7za.dll"));

SevenZipCompressor compressor = new SevenZipCompressor();

compressor.Compressing += Compressor_Compressing;
compressor.FileCompressionStarted += Compressor_FileCompressionStarted;
compressor.CompressionFinished += Compressor_CompressionFinished;

string password = @"whatever";
string destinationFile = @"C:\Temp\whatever.zip";
string[] sourceFiles = Directory.GetFiles(@"C:\Temp\YourFiles\");

if (String.IsNullOrWhiteSpace(password))
{
    compressor.CompressFiles(destinationFile, sourceFiles);
}
else
{
    //optional
    compressor.EncryptHeaders = true;
    compressor.CompressFilesEncrypted(destinationFile, password, sourceFiles);
}

I want to add some more alternatives.

For .NET one can use SharpZipLib, for Xamarin use SharpZipLib.Portable.

Example for .NET:

using ICSharpCode.SharpZipLib.Zip;

// Compresses the supplied memory stream, naming it as zipEntryName, into a zip,
// which is returned as a memory stream or a byte array.
//
public MemoryStream CreateToMemoryStream(MemoryStream memStreamIn, string zipEntryName) {

    MemoryStream outputMemStream = new MemoryStream();
    ZipOutputStream zipStream = new ZipOutputStream(outputMemStream);

    zipStream.SetLevel(3); //0-9, 9 being the highest level of compression
    zipStream.Password = "Your password";

    ZipEntry newEntry = new ZipEntry(zipEntryName);
    newEntry.DateTime = DateTime.Now;

    zipStream.PutNextEntry(newEntry);

    StreamUtils.Copy(memStreamIn, zipStream, new byte[4096]);
    zipStream.CloseEntry();

    zipStream.IsStreamOwner = false;    // False stops the Close also Closing the underlying stream.
    zipStream.Close();          // Must finish the ZipOutputStream before using outputMemStream.

    outputMemStream.Position = 0;
    return outputMemStream;

    // Alternative outputs:
    // ToArray is the cleaner and easiest to use correctly with the penalty of duplicating allocated memory.
    byte[] byteArrayOut = outputMemStream.ToArray();

    // GetBuffer returns a raw buffer raw and so you need to account for the true length yourself.
    byte[] byteArrayOut = outputMemStream.GetBuffer();
    long len = outputMemStream.Length;
}

More samples can be found here.

If you can live without password functionality, one can mention ZipStorer or the built in .NET function in System.IO.Compression.

DotNetZip worked great in a clean way.

DotNetZip is a FAST, FREE class library and toolset for manipulating zip files.

Code

static void Main(string[] args)
{
        using (ZipFile zip = new ZipFile())
        {

            zip.Password = "mypassword";

            zip.AddDirectory(@"C:\Test\Report_CCLF5\");
            zip.Save(@"C:\Test\Report_CCLF5_PartB.zip");
        }
 }