Specified cast is not valid while reading a TIFF file

[vsolominov]

Summary of the issue

While reading a TIFF file, an error about the incorrect type occurs.

Environment

iTextSharp.LGPLv2.Core version: 3.4.7
.NET Core SDK version: 6.0
IDE: Visual Studio 2022

Example code/Steps to reproduce:

I'm trying to read a TIFF file like this:

var randomAccessor = new RandomAccessFileOrArray(input);
var pageCount = TiffImage.GetNumberOfPages(randomAccessor);
for (int pageIndex = 1; i <= pageCount; pageIndex++)
{
    var image = TiffImage.GetTiffImage(randomAccessor, pageIndex);
}

Problem when calling the GetField(tag) method on a tag that is not in the dictionary:

var fillOrderField = dir.GetField(TiffConstants.TIFFTAG_FILLORDER);
if (fillOrderField != null)
{
    fillOrder = fillOrderField.GetAsInt(0);
}

If we turn to the implementation, then if there is no tag in the _fieldIndex dictionary, the value 0 will be returned. Then _fields[0] is called, resulting in an incorrect value, without GetField(tag) returning null.

Output:

Exception message: Specified cast is not valid.
Full Stack trace:
  at iTextSharp.text.pdf.codec.TiffField.GetAsInt(Int32 index) in G:\iTextSharp.LGPLv2.Core\src\iTextSharp.LGPLv2.Core\iTextSharp\text\pdf\codec\TIFFField.cs:line 310
  at iTextSharp.text.pdf.codec.TiffImage.GetTiffImageColor(TiffDirectory dir, RandomAccessFileOrArray s) in G:\iTextSharp.LGPLv2.Core\src\iTextSharp.LGPLv2.Core\iTextSharp\text\pdf\codec\TiffImage.cs:line 443
  at iTextSharp.text.pdf.codec.TiffImage.GetTiffImage(RandomAccessFileOrArray s, Int32 page, Boolean direct) in G:\iTextSharp.LGPLv2.Core\src\iTextSharp.LGPLv2.Core\iTextSharp\text\pdf\codec\TiffImage.cs:line 110
  at iTextSharp.text.pdf.codec.TiffImage.GetTiffImage(RandomAccessFileOrArray s, Int32 page) in G:\\iTextSharp.LGPLv2.Core\src\iTextSharp.LGPLv2.Core\iTextSharp\text\pdf\codec\TiffImage.cs:line 77
  at Pdf.Program.Execute(String input, String output) in G:\Pdf\Program.cs:line 129