How can you read for a specific value from a binary file starting from the end of the file?

I am trying to figure out how to actually (appropriately) read for the PDF trailer Byte_offset_of_last_cross-reference_section from a PDF file.

According to the PDF 1.7 (ISO 32000-1:2008) specification, the file structure is designed in a way that it should be read from the end of the file. Here is an example of what a simplified (minimal) trailer looks like when I use a StreamReader and read the file line-by-line (UTF8 Encoding):

trailer
<< key1 value1
     key2 value2
     …
     keyn valuen
>>
startxref
Byte_offset_of_last_cross-reference_section
%%EOF

trailer
<</Root 7 0 R /Size 7>>
startxref
696
%%EOF

The value I want to somehow grab is the 696 value. I'm just not sure how to do that using a BinaryReader starting from the end of the file.

Solution

You can use the Seek method, see here for examples. You can use SeekOrigin.End as argument, see here for other options

example:

using (var reader = File.Open(...))
{
    reader.Seek(100, SeekOrigin.End);
    //...
}

You can start reading backwards in a loop till you get to the startxref marker (or anything that helps you know that you can read 696) or assume a length of 100 bytes from the end of the file and then do a lookup in that small array as Anthony suggested in the comment below.