How to return result with empty value?

Question

C++ / C#

There is a function that accepts a value that will be checked against the value from the entity.

public async Task<long?> GetId(long bookingId)
{
      var bookPriceDb = await context.BookPrice.SingleOrDefaultAsync(x => x.BookingId == bookingId);
      var Id = bookingId;

      return Id;
}

There is a possibility that the values ​​will not be found => what to do with the check:

if(bookPriceDb == null)
{
???
}

Answer 1

What to guess, you can clearly return found / not found

public async Task<(bool, long)> GetId(long bookingId)

Answer 2

How does your architecture imply error handling?
Throw an exception. Return Nan/Infinity/-1. Return not long, but something that can be nullable (although it seems like from some kind of sisharp this can be stirred up with simple types)

Answer 3

Usually in such cases, an error is thrown and handled higher in the code, or in place if the value is null.

if(bookPriceDb == null)
{
 throw new NullReferenceException ("не существует указанного id: {bookingId} ")
}