LINQ：将集合中的所有时间跨度相加/求和

Question

I'm trying to simply add up all the TimeSpans in my ViewModel, but I keep getting an error from this (and I can assure you that 'x.totalDuration' is in fact a TimeSpan and that 'myCollection' is indeed an IEnumerable:

View:

  var myCollection = Model.Select(x => x.totalDuration);
  var ts = new TimeSpan(myCollection.Sum(r => r.Ticks)); <-- boom here--<

Controller:

 [HttpGet]
    public ActionResult List()
    {
        var model = _db.Tracks
          .Where(r => r.UserID == WebSecurity.CurrentUserId)
          .Select(x => new iOSHistoryListViewModel
          {
              averagePace = Math.Round(26.8224 / x.locations.Average(y => y.speed), 2),
              createdDate = x.createdDate,
              FirstName = x.UserProfile.FirstName,
              totalDistance = Math.Round(x.totalDistance / 1609.344, 2),
              totalDuration = TimeSpan.FromSeconds(x.totalDuration),
              trackId = x.TrackID
          })
          .OrderByDescending(x => x.createdDate)
          .ToList(); <-- tried with/without this

        return View(model);
    }

Error:

LINQ to Entities does not recognize the method 'System.TimeSpan FromSeconds(Double)' method, and this method cannot be translated into a store expression.

Description: An unhandled exception occurred during the execution of the     current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NotSupportedException: LINQ to Entities does not recognize the method 'System.TimeSpan FromSeconds(Double)' method, and this method cannot be translated into a store expression.

Source Error: 


Line 13:         @{
Line 14:             var myCollection = Model.Select(x => x.totalDuration);
Line 15:             var ts = new TimeSpan(myCollection.Sum(r => r.Ticks));

This is so simple, yet I am missing something...

Answer 1

The problem is you are trying to call TimeSpan.FromSeconds in the database. This is a .NET method and can't be translated. This is untested, but to fix try this:

        var model = _db.Tracks
      .Where(r => r.UserID == WebSecurity.CurrentUserId)
      .Select(x => new
      {
          averagePace = Math.Round(26.8224 / x.locations.Average(y => y.speed), 2),
          createdDate = x.createdDate,
          FirstName = x.UserProfile.FirstName,
          totalDistance = x.totalDistance,
          totalDuration = x.totalDuration,
          trackId = x.TrackID
      }).ToList()
      .Select(x => new iOSHistoryListViewModel{
          averagePace = x.averagePace,
          createdDate = x.createdDate,
          FirstName = x.FirstName,
          totalDistance = Math.Round(x.totalDistance / 1609.344, 2),
          totalDuration = TimeSpan.FromSeconds(x.totalDuration),
          trackId = x.trackId
      })
      .OrderByDescending(x => x.createdDate)
      .ToList();

The key here is that the first Select throws in the data returned from the data source into a list. The list is then in .NET memory where you can do any .NET operations. Then send that result set to a new list as well. That should get rid of your exception about .FromSeconds