c# - 为什么 LINQ 查询会在我尝试获取某种类型的计数时抛出异常

Question

public readonly IEnumerable<string> PeriodToSelect = new string[] { "MONTH" };  

var dataCollection = from p in somedata    
from h in p.somemoredate    
where h.Year > (DateTime.Now.Year - 2)    
where PeriodToSelect.Contains(h.TimePeriod)  
select new  
{  
    p.Currency, 
    h.Year.Month, h.Value                                                    
}; 

有人能告诉我为什么在下面的代码行会抛出异常吗？

int count = dataCollection.Count();  

这是异常(exception):

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Linq.Enumerable.<SelectManyIterator>d__31`3.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__31`3.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at ...

Answer 1

这看起来像是 linq2objects 中的正常空引用异常，它试图执行您的谓词或投影。

如果“somedata”集合的某些元素为空，如果“h.Year”为空(那是什么类型？)，或者如果“p.somemoredate”为空..