LINQ中檢查可能的空值

Question

我有一個SQL數據庫建模這個類。

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 

我想語言的列表，也是最後一篇文章

的日期，但我要檢查是否有物品的物品，如果物品有一個有效日期。

查詢的正確方法是什麼？我到目前爲止已經做到了這一點：

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

我認爲Article類看起來像：

Public Class Article 
    Public Property Dated as DateTime? 
End Class 

你應該使用EF從數據庫中獲得DateTime?，然後執行ToShortDateString作爲LINQ到對象查詢。 EF將無法將ToShortDateString轉換爲正確的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()) 
       }