GZipStream length when uncompressed

Introduction

As stated in the documentation, the Property Length of the GZipStream is not supported. so you wont really know the size of the file, your about to extract.

Using the code

/// <summary>
/// Extracts the original filesize of the compressed file.
/// </summary>
/// <param name="fi">GZip file to handle</param>
/// <returns>Size of the compressed file, when its decompressed.</returns>
/// <remarks>More information at <a href="http://tools.ietf.org/html/rfc1952">http://tools.ietf.org/html/rfc1952</a> section 2.3</remarks>
public static int GetGzOriginalFileSize(string fi)
{
    return GetGzOriginalFileSize(new FileInfo(fi));
}
/// <summary>
/// Extracts the original filesize of the compressed file.
/// </summary>
/// <param name="fi">GZip file to handle</param>
/// <returns>Size of the compressed file, when its decompressed.</returns>
/// <remarks>More information at <a href="http://tools.ietf.org/html/rfc1952">http://tools.ietf.org/html/rfc1952</a> section 2.3</remarks>
public static int GetGzOriginalFileSize(FileInfo fi)
{
    try
    {
        using (FileStream fs = fi.OpenRead())
        {
            try
            {
                byte[] fh = new byte[3];
                fs.Read(fh, 0, 3);
                if (fh[0] == 31 && fh[1] == 139 && fh[2] == 8) //If magic numbers are 31 and 139 and the deflation id is 8 then...
                {
                    byte[] ba = new byte[4];
                    fs.Seek(-4, SeekOrigin.End);
                    fs.Read(ba, 0, 4);
                    return BitConverter.ToInt32(ba, 0);
                }
                else
                    return -1;
            }
            finally
            {
                fs.Close();
            }
        }
    }
    catch (Exception)
    {
        return -1;
    }
}

Its pretty simple you just call the GetGzOriginalFileSize method and parse the file path or fileinfo object and you get the size of the file when its decompressed.

Resources

  C# and I

History

  16-08-2013: First post.