加入兩個LINQ結果

Question

我有兩個LINQ的結果（結果，結果2）

 List<Employee> result= new List<Employee> { 
      new Employee { EmpId= 4, Date ='2014-02-26', EmpName= "abc"}, 
      new Employee { EmpId = 5, Date = '2014-02-26', EmpName= "abc"}, 
      new Employee { EmpId = 6, Date ='2014-02-26', EmpName= "abc"}, 
      new Employee { EmpId= 7, Date = '2014-02-26', EmpName= "abc"} }; 

     // Create a list of students. 
     List<Student> result2 = new List<Student> { 
      new Student { EmpId= 1, Date ='2014-02-26', StudentId = 9562 }, 
      new Student { EmpId= 2,Date ='2014-02-26', StudentId = 9870 }, 
      new Student { EmpId= 4,Date ='2014-02-26', StudentId = 9913 } }; 

我想下面的結果，我曾與各種查詢嘗試，但並沒有得到這樣的：

new Employee { EmpId= 4, Date ='2014-02-26', EmpName= "abc",StudentId=9913}, 
      new Employee { EmpId = 5, Date = '2014-02-26', EmpName= "abc",StudentId=Null}, 
      new Employee { EmpId = 6, Date ='2014-02-26', EmpName= "abc",StudentId=Null}, 
      new Employee { EmpId= 7, Date = '2014-02-26', EmpName= "abc",StudentId=Null} }; 

我已經執行了下面的查詢，但得到一個異常「

（NullReferenceException被捕獲）未將對象引用設置爲對象的實例。

var res = (from r in result 
          join r2 in result2 on new { EmpId= r.EmpId, Date = r.Date.Value.Year } equals new { EmpId= Convert.ToInt32(r2.EmpId), Date = r2.Date.Value.Year } into list 
          from l in list.DefaultIfEmpty() 
          select new 
          { 
           EmpId= r.EmpId, 
    Date=r.Date, 
    EmpName=r.EmpName, 
    StudentId=l.StudentId 
    }).ToList(); 

請讓我知道，如果有人知道。

Answer 1

的問題是在這裏：

StudentId = l.StudentId 

你的一些l值都null所以NullReferenceException被拋出。您需要檢查null值：

var res = (from r in result 
      join r2 in result2 
       on new { EmpId= r.EmpId, Date = r.Date.Value.Year } 
       equals new { EmpId= Convert.ToInt32(r2.EmpId), Date = r2.Date.Value.Year } 
       into list   
      from l in list.DefaultIfEmpty() 
      select new 
      { 
       EmpId = r.EmpId, 
       Date = r.Date, 
       EmpName = r.EmpName, 
       StudentId = (l != null) ? l.StudentId : (int?)null 
      }).ToList(); 

（考慮LINQ到對象;不知道如果這樣的表達在LINQ到實體是有效的）