Reading and writing nested Zip archives

The following examples show how to use the ZipReader.GetItemDataStream and ZipWriter.GetItemDataStream methods to allow the component to easily handle nested zip archives.

A nested zip archive is when an item in an archive is another zip archive. By providing the stream returned by the method to a new instance of ZipWriter or ZipReader, a nested zip file will be created or read.

The first example shows how the ZipWriter.GetItemDataStream() method can be used to create a nested Zip archive.

private void NestedArchiveExample( Stream someStream )

{

  using( ZipWriter writer = new ZipWriter( someStream ) )

  {

    ZipItemLocalHeader header = new ZipItemLocalHeader();



    header.FileName = "File1.zip";

    writer.WriteItemLocalHeader( header );



    // Create a write stream that wraps the item's data

    using( Stream itemStream = writer.GetItemDataStream() )

    {

      // Feed the item stream to a new instance of ZipWriter

      using( ZipWriter nestedWriter = new ZipWriter( itemStream ) )

      {

        /* The 'using' statement will insure the nested zip file is properly terminated */



        // Add an item and some data into the nested zip file

        header.FileName = "File1.dat";

        nestedWriter.WriteItemLocalHeader( header );

        nestedWriter.WriteItemData( MediumFile1 );

      }

    }

  }

}

VB.NET

Private Sub NestedArchiveExample(ByVal someStream As Stream)

  Using writer As New ZipWriter(someStream)

    Dim header As New ZipItemLocalHeader()



    header.FileName = "File1.zip"

    writer.WriteItemLocalHeader(header)



    ' Create a write stream that wraps the item's data

    Using itemStream As Stream = writer.GetItemDataStream()

      ' Feed the item stream to a new instance of ZipWriter

      Using nestedWriter As New ZipWriter(itemStream)

        ' The 'using' statement will insure the nested zip file is properly terminated



        ' Add an item and some data into the nested zip file

        header.FileName = "File1.dat"

        nestedWriter.WriteItemLocalHeader(header)

        nestedWriter.WriteItemData(MediumFile1)

      End Using

    End Using

  End Using

End Sub

The second example shows how the ZipReader.GetItemDataStream method can be used to read a nested Zip archive.

private void ReadZipArchive( Stream archiveStream )

{

  using( ZipReader reader = new ZipReader( archiveStream ) )

  {

    ZipItemLocalHeader header;



    while( ( header = reader.ReadItemLocalHeader() ) != null )

    {

      /* The component will not automatically identity what is and is

      * not a nested zip file. The application needs to have its own mechanism.

      *

      * This example will keep it simple and just look at the item's file name extension. */



      // If the item's extension is .zip

      if( StringComparer.OrdinalIgnoreCase.Compare( Path.GetExtension( header.FileName ), ".zip" ) == 0 )

      {

        // Create a read stream that wraps the item's data

        using( Stream itemStream = reader.GetItemDataStream() )

        {

          /* NOTE: While using a recursive call here makes for elegant and compact code that

          * helps illustrate the concept of nested zip archives, a maliciously crafted zip file

          * made up of a large number of nested zip files could make this code cause a stack overflow

          * due to excessive recursion. */



          // Call ourselves to read the nested archive

          this.ReadZipArchive( itemStream );

        }



        /* IMPORTANT: Before we can move on to the next item in the archive, ZipReader must have

        * reached the end of the current item's data. Reading a nested zip file does not ensure

        * this, so we need to make sure here by reading any remaining data into a dummy stream.

        *

        * Failure to do this will result in a ZipReaderException that reports that the object

        * is not in the correct state to read the next item header. */



        // Make sure we reach the end of the item's data

        reader.ReadItemData( Stream.Null );

      }

      else

      {

        // This example does not concern itself with normal items

        reader.ReadItemData( Stream.Null );

      }

    }

  }

}

VB.NET

Private Sub ReadZipArchive(ByVal archiveStream As Stream)

  Using reader As New ZipReader(archiveStream)

    Dim header As ZipItemLocalHeader



    header = reader.ReadItemLocalHeader()

    Do While header IsNot Nothing

'      The component will not automatically identity what is and is

'      not a nested zip file. The application needs to have its own mechanism.

'     

'      This example will keep it simple and just look at the item's file name extension.



      ' If the item's extension is .zip

      If StringComparer.OrdinalIgnoreCase.Compare(Path.GetExtension(header.FileName), ".zip") = 0 Then

        ' Create a read stream that wraps the item's data

        Using itemStream As Stream = reader.GetItemDataStream()

'          NOTE: While using a recursive call here makes for elegant and compact code that

'          helps illustrate the concept of nested zip archives, a maliciously crafted zip file

'          made up of a large number of nested zip files could make this code cause a stack overflow

'          due to excessive recursion.



          ' Call ourselves to read the nested archive

          Me.ReadZipArchive(itemStream)

        End Using



'        IMPORTANT: Before we can move on to the next item in the archive, ZipReader must have

'        reached the end of the current item's data. Reading a nested zip file does not ensure

'        this, so we need to make sure here by reading any remaining data into a dummy stream.

'       

'        Failure to do this will result in a ZipReaderException that reports that the object

'        is not in the correct state to read the next item header.



        ' Make sure we reach the end of the item's data

        reader.ReadItemData(Stream.Null)

      Else

        ' This example does not concern itself with normal items

        reader.ReadItemData(Stream.Null)

      End If

      header = reader.ReadItemLocalHeader()

    Loop

  End Using

End Sub