附件解密系统.FormatException: Base-64字符数组或字符串的长度无效

我有一个c#代码，它采用infopath中附件的加密编码字符串并对其解密。这是下面的代码:

    private int fileSize;
    private int attachmentNameLength;
    private string attachmentName;
    private byte[] decodedAttachment;

    /// <summary>
    /// Accepts the Base64 encoded string
    /// that is the attachment.
    /// </summary>
    public InfoPathAttachmentDecoder(string theBase64EncodedString)
    {
        **byte[] theData =     Convert.FromBase64String(theBase64EncodedString);** //This line throws a   System.FormatException: Invalid length for a Base-64 char array or string.
        using (MemoryStream ms = new MemoryStream(theData))
        {
            BinaryReader theReader = new BinaryReader(ms);
            DecodeAttachment(theReader);
        }
    }
    private void DecodeAttachment(BinaryReader theReader)
    {
        //Position the reader to obtain the file size.
        byte[] headerData = new byte[FIXED_HEADER];
        headerData = theReader.ReadBytes(headerData.Length);
        fileSize = (int)theReader.ReadUInt32();
        attachmentNameLength = (int)theReader.ReadUInt32() * 2;
        byte[] fileNameBytes = theReader.ReadBytes(attachmentNameLength);
        //InfoPath uses UTF8 encoding.
        Encoding enc = Encoding.Unicode;
        attachmentName = enc.GetString(fileNameBytes, 0,  attachmentNameLength - 2);
        decodedAttachment = theReader.ReadBytes(fileSize);
    }
    public void SaveAttachment(string saveLocation)
    {
        string fullFileName = saveLocation;
        if (!fullFileName.EndsWith(Path.DirectorySeparatorChar.ToString()))
        {
            fullFileName += Path.DirectorySeparatorChar;
        }
        fullFileName += attachmentName;
        if (File.Exists(fullFileName))
            File.Delete(fullFileName);
        FileStream fs = new FileStream(fullFileName, FileMode.CreateNew);
        BinaryWriter bw = new BinaryWriter(fs);
        bw.Write(decodedAttachment);
        bw.Close();
        fs.Close();
    }
    public string Filename
    {
        get { return attachmentName; }
    }
    public byte[] DecodedAttachment
    {
        get { return decodedAttachment; }
    }
}

line byte[] theData = Convert.FromBase64String(theBase64EncodedString);抛出系统格式异常，base64数组或字符串的长度无效。