A class library for discovering the format of files. Useful when receiving data from end-users when you cannot always rely on the content type specified (or trust it).

The library is extensible, it can support further formats.

Stable builds are available as NuGet packages. You can install it via the Package Manager or via the Package Manager Console:

Install-Package Workshell.FileFormats

You can call FileFormat.Get(string) with a file name to attempt to disocver the format of a file, such as:

var format = FileFormat.Get(@"C:\Windows\explorer.exe");

You can also call FileFormat.Get(Stream) on a stream, such as:

var file = new FileStream(@"C:\Windows\explorer.exe", FileMode.Open, FileAccess.Read);
var format = FileFormat.Get(file);

Note that the Stream instance should support seeking as the stream will need to be rewound during the scanning process.

If a format could not be found then a simple null will be returned. If it is found then you can test the return type directly, such as:

if (format is PortableExecutableFormat)
{
    // Do something...
}

Formats can also take advantage of inheritance, for example the Office Open XML Word Document format has the following chain:

WordDocumentFormat -> OfficeZipFormat -> ZipFormat -> FileFormat

This means you can check further up the chain. For example:

if (format is ZipFormat)
{
    // Do something...
}

The above example would cover any file that's detected as being ZIP based, which includes Office Open XML and the Open Document Format.

In order to add support for a new format you first need to define a subclass of FileFormat which defines the content types and file extensions of matched data.

Take for example the PDF format:

public class PDFFormat : FileFormat
{
    private static readonly string[] _contentTypes => new[] 
    { 
        "application/pdf",
        "application/x-pdf"
    };
    private static readonly string[] _extensions => new[] { "pdf" };

    public PDFFormat() : base(_contentTypes, _extensions)
    {
    }

    public override int SortIndex => 10;
}

You will also note the SortIndex property. This is useful for files that count as two different types.

For example, Office Open XML file types are based on ZIP files, so they are both ZIP files and Office files. When scanning we want the OOXML format to rank higher than the ZIP format, so we give it a higher sort index.

Once you have your FileFormat class you then need to subclass FileFormatScanner which is used to perform the actual data scanning.

The FileFormatScanner class looks like this:

public abstract class FileFormatScanner
{
    public abstract FileFormat Match(FileFormatScanJob job);
}

It only has one method you need to override, FileFormatScanner.Match(FileFormatScanJob).

The FileFormatScanJob instance supplied to the method contains the first and last 4KB of the data being scanned and a stream. You use these to perform the analysis required to determine the format of the data.

The 7-Zip scanner for example looks like this:

public class SevenZipFormatScanner : FileFormatScanner
{
    private static readonly byte?[] Signature = new byte?[] { 0x37, 0x7A, 0xBC, 0xAF, 0x27, 0x1C };

    public SevenZipFormatScanner()
    {
    }

    public override FileFormat Match(FileFormatScanJob job)
    {
        if (FileFormatUtils.IsNullOrEmpty(job.StartBytes))
            return null;

        if (job.StartBytes.Length <= Signature.Length)
            return null;

        if (!FileFormatUtils.MatchBytes(job.StartBytes, Signature))
            return null;

        var fingerprint = new SevenZipFormat();

        return fingerprint;
    }
}

Take a look at the existing formats for reference implementations.

If you think there's a common format we should cover then please do let us know and we'll try and add support for it.

We currently use a modified and internalised variant of OpenMCDF for reading some file formats, especially legacy Microsoft Office files.

The original version of OpenMCDF is available here: https://github.com/ironfede/openmcdf

The code remains copyright and licensed to those respective authors.

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