LINQ - 連接兩個表和的右側部分獲得的值加入，如果它存在拋出異常

Question

我有問題的表和答案的表

我總是需要，無論顯示的問題清單和有一個corresponsing asnwer，我要抓住這個問題的答案（響應）

我用下面的代碼

var Questions = db.Questions.Where(x => x.isActive); 
var Answers = db.Answers.Where(x => x.AssessmentID == 99); 

AssessmentResponseVM model = new AssessmentResponseVM(); 
foreach (var question in Questions) 
{ 
    AnswerAndQuestions q = new AnswerAndQuestions { }; 
    q.QuestionText = question.QuestionText; 
    q.QuestionID = question.ID; 
    q.Response = Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault().Response; <--- throws exception if there is no answer for the question 
    model.Questions.Add(q); 
} 

但得到這個錯誤

Object reference not set to an instance of an object. 

在此行中

q.Response = Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault().Response; 

Answer 1

變化

q.Response = Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault().Response; 

這個代碼

q.Response=Answers.Any(a=>a.QuestionID==question.ID)?Answers.firstOrDefault(a => a.QuestionID == question.ID).Response:new Response(); 

Answer 2

q.Response = Answers.Any(a => a.QuestionID == question.ID) ? Answers.Where(a => a.QuestionID == question.ID).FirstOrDefault().Response; 

Answer 3

如果你允許使用C＃6.0中，我會建議嘗試新的null-conditional operator。

q.Response = Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault()?.Response; 

它返回null時Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault()返回默認值，即空。

如果您需要別的東西，而不是null，您可以使用null-coalescing operator。

q.Response = Answers.Where(a => a.QuestionID == question.ID).SingleOrDefault()?.Response ?? response, what you need;