Applying the concept of VB Module in C#

So I used to write my functions in VB module and call them from any  place in the project, I don't know how to do it in C#.

This is my current function in the controller

private readonly ApplicationDbContext _context;

        public PatRegController(ApplicationDbContext context)
        {
            _context = context;
        }

  public Int64 GetSerial()
        {
            List<FileIdSeq> NewFileSeq = new List<FileIdSeq>();
            NewFileSeq = _context.FileIdSeq.FromSql("FileIdSeqSP").ToList();
            var FileID = NewFileSeq[0].LastSequence;
            return FileID;
        }

and I call it 

GetSerial()

Now I moved it to another static class so I can initialize it from any place as follows

namespace ARTNetCore.Functions
{
    public static class DbSerializers
    {
        private static readonly ApplicationDbContext _context;
        public static Int64 FileIdSerial() {
            List<FileIdSeq> NewFileSeq = new List<FileIdSeq>();
            NewFileSeq = _context.FileIdSeq.FromSql("FileIdSeqSP").ToList();
            var FileID = NewFileSeq[0].LastSequence; return FileID;
        }
    }
}

and I call it in my controller like,

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create([Bind("FName,MName,LName,Dob,GenrId,StasId,NatyId,MarsId,CouyId,StaeId,CityId,OccnId,Email,SNN,PassNo,MobNo,LLine,MAdds,StrtNo,SDirn,AptNo,Locy,ALevl,PCode,Couy,ProeId")] PatReg patReg)
        {
            if (ModelState.IsValid)
            {
                patReg.FileId = DbSerializers.FileIdSerial();
                _context.Add(patReg);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(patReg);
        }

But I m getting an error 

An unhandled exception occurred while processing the request.

NullReferenceException: Object reference not set to an instance of an object.
 NewFileSeq = _context.FileIdSeq.FromSql("FileIdSeqSP").ToList();

what am i missing here