LINQ列表給出了空例外條款

Question

var q = from p in query 
where 
    ((criterias.birthday == p.BirthDay|| criterias.birthday == null))         
&& ((criterias.marriageDate == null || criterias.marriageDate == p.MarriageDate))         
&& ((criterias.gender == p.Gender) || (criterias.gender == null)) 
&& ((criterias.nationalities.Contains(p.Nationality)) || (criterias.nationalities == null)) 

標準是一個類，我存儲我的搜索標準。國籍是一個字符串列表。當我在字符串中沒有項目時會發生問題。該查詢會引發空引用異常。該查詢在國籍中不接受空值。我怎樣才能解決這個問題？使空校驗來查詢之前

Answer 1

顛倒順序：如你使用||，表達的第二部分只計算在第一部分的計算結果爲假：

&& ((criterias.nationalities == null) || 
      (criterias.nationalities.Contains(p.Nationality))) 

Answer 2

試換國籍檢查的順序。在嘗試評估Contains之前，它應該在空檢查上短路。

((criterias.nationalities == null) || (criterias.nationalities.Contains(p.Nationality))) 

Answer 3

打開這個說法周圍：

(criterias.nationalities.Contains(p.Nationality)) || (criterias.nationalities == null) 

以便讀取

(criterias.nationalities == null) || (criterias.nationalities.Contains(p.Nationality)) 

如果第一個操作數的計算結果爲真，第二個將被跳過。

Answer 4

看着這些2：

((criterias.birthday == p.BirthDay|| criterias.birthday == null)) 
&& ((criterias.marriageDate == null || criterias.marriageDate == p.MarriageDate)) 

我不認爲marriageDate會給你的問題，但生日使用錯誤的順序。
在這種情況下，你需要的||的「短路計算」屬性，將其更改爲：

(criterias.birthday == null || criterias.birthday == p.BirthDay) 
&& (criterias.marriageDate == null || criterias.marriageDate == p.MarriageDate) 

Answer 5

嘗試使用無效首先檢查，然後（如果不爲空），檢查包括：

var q = from p in query 
where  
((criterias.birthday == p.BirthDay|| criterias.birthday == null))         
&& ((criterias.marriageDate == null || criterias.marriageDate == p.MarriageDate)) 
&& ((criterias.gender == p.Gender) || (criterias.gender == null)) 
&& ((criterias.nationalities == null) || (criterias.nationalities.Contains(p.Nationality))