Linq查询以获取最后一条记录

Question

I know this is asked many times and I've searched most of the solutions online, but nothing seems to make it for me. I have a table with this structure:

ID | ScheduleId | Filename | Description
 1 |     10     |          |  ....
 2 |     10     | test.txt |  .....

I want to get the last non-empty Filename by passing the ScheduleId(eg to get "test.txt" in this case).

I've tried many things and nothing seems to get me the Filename. Here is the last one:

var tempFileName = objContext.SchedulesAndFiles
                           .Where(x => x.ScheduleId == scheduleId)
                           .OrderByDescending(x => x.ScheduleId)
                           .Take(1).Select(x => x.Filename);

This doesn't work as well, although I understand why it doesn't:

var tempFileName = from e in objContext.SchedulesAndFiles
                   where e.ScheduleId == scheduleId 
                   orderby e.ScheduleId descending
                   select e.Filename;

Calling .Last() or .LastOrDefault() throws an exception( The query operator 'LastOrDefault' is not supported. )

Answer 1

if have to include that you want only non-empty filenames. You may also use ToList() to finalize the query, then FirstOrDefault() should work as expected, try

var tempFileName = objContext.SchedulesAndFiles
                             .Where(x 
                                 => x.ScheduleId == scheduleId 
                                 && x.Filename != null 
                                 && x.Filename != "")
                             .OrderByDescending(x => x.ScheduleId)
                             .Take(1)
                             .Select(x => x.Filename)
                             .ToList()
                             .FirstOrDefault();

Answer 2

You should sort your records based on the ID instead of ScheduleId and also filter the records that has the empty Filename :

objContext.SchedulesAndFiles
          .Where(x => x.ScheduleId == scheduleId && x.Filename != "")
          .OrderByDescending(x => x.ID)
          .First().Filename;

Answer 3

You can try this query. I think you have to issue the last or default before selecting the file name

 var tempFileName = objContext.SchedulesAndFiles
                               .Where(x => x.ScheduleId == scheduleId && ! string.IsNullOrEmpty(e.FileName))
                               .FirstOrDefault().Select(x => x.Filename);

Answer 4

One option is to call ToList() or AsEnumerable() before trying to use LastOrDefault().

var tempFileName = objContext.SchedulesAndFiles
                   .Where(x => x.ScheduleId == scheduleId 
                            && x.Filename != null && x.Filename != '')
                   .ToList().LastOrDefault();
if(tempFileName != null)
{
    // Do something
}

Answer 5

The final attempt:

var tempFileName = objContext.SchedulesAndFiles
                         .Where(x 
                             => x.ScheduleId == scheduleId 
                             && x.Filename != null 
                             && x.Filename != "")
                         .OrderByDescending(x => x.ID)
                         .First()
                         .Select(x => x.Filename);

For every item with this scheduleId, this gets the all of the items which have a non-empty fileName, sorted descending by ID (assuming that higher IDs are inserted after lower IDs), getting the First() (should be supported) and getting its FileName.

Note that you could run into a NullPointerException on First() if there is no satisfying fileName.

Also, you may need to normalize/trim to not find spaces/tabs and such things.