LINQ中检查可能的空值

Question

我有一个SQL数据库建模这个类。

Public Class Language 
    Public Property Name As String 
    Public Property Articles As List(Of Article) 
End Class 

Public Class Article 
    Public Property Dated as DateTime? 
End Class 

我想语言的列表，也是最后一篇文章

的日期，但我要检查是否有物品的物品，如果物品有一个有效日期。

查询的正确方法是什么？我到目前为止已经做到了这一点：

Dim query = From x In context.Languages 
       Order By x.Name 
       Select New MyViewModel() With { 
       .Name = x.Name, 
       .LastArticleDate = If(x.Articles.Count <> 0 AndAlso 
        x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault IsNot Nothing AndAlso 
        x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault.Dated.HasValue, 
        x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault.Dated.Value.ToShortDateString, 
        String.Empty) 
       } 

Answer 1

我认为Article类看起来像：

Public Class Article 
    Public Property Dated as DateTime? 
End Class 

你应该使用EF从数据库中获得DateTime?，然后执行ToShortDateString作为LINQ到对象查询。 EF将无法将ToShortDateString转换为正确的SQL。

Dim query = From x In context.Languages 
      Order By x.Name 
      Select New With { 
       .Name = x.Name, 
       .LastArticleDate = x.Articles.OrderByDescending(Function(x) x.Dated).FirstOrDefault() 
      } 

Dim results = From x In query.AsEnumerable() 
       Select New MyViewModel() With { 
       .Name = x.Name, 
       .LastArticleDate = If(!x.LastArticleDate.HasValue, "", x.LastArticleDate.ToShortDateString()) 
       }