linq sum source为空的两种处理方式

 total = query.Select(l => l.Price).DefaultIfEmpty(0m).Sum();

 // Exceptions:
 //   System.ArgumentNullException:

 //     source is null.

  total = (from tb in query select tb.Price).Any() ? (from tb in query select tb.Price).Sum() : 0m;

或者

 total = query.Select(l => l.Price).DefaultIfEmpty(0m).Sum();