linq加入兩個id與單個列表的SQL語句

Question

我想用我的記錄列表連接用戶列表，但是記錄列表有兩列，我應該使用用戶列表的值，這兩列中的一列也是可以爲空的。如何正確加入？我試圖做這樣的事情：

var results = (from r in records 
       join u in users on r.RegisteredBy equals u.Id 
       join u in users on r.ModifiedBy equals u.Id 
       select new CustomResult() 
       { 
        Id = r.Id, 
        Name = r.Name, 
        RegisteredByName = u.Name, 
        ModifiedByName = u.Name 
       }).ToList(); 

如我所料它沒有工作，我記得我必須將其設置爲空，如果使用默認值。

比如我有用戶

var user1 = new User() { Id = 1, Name = "John" }; 
var user2 = new User() { Id = 2, Name = "Matt" } 
var user3 = new User() { Id = 3, Name = "George" }; 

List<User> users = new List<User>(){ user1, user2, user3 }; 

的列表，我有我的記錄另一個列表，即

var record1 = new Record() { Id = 1, Name = "Record1", RegisteredBy = 1, ModifiedBy = 3}; 
var record2 = new Record() { Id = 2, Name = "Record2", RegisteredBy = 3, ModifiedBy = null }; 
var record3 = new Record() { Id = 3, Name = "Record3", RegisteredBy = 2, ModifiedBy = 1 }; 

List<Record> records = new List<Record>(){ record1, record2, record3 }; 

由於這一結果加入我要讓另一個類的列表，有我需要的信息，即

var result1 = new CustomResult(){ Id = 1, Name = "Record1", RegisteredByName = "John", ModifiedByName = "George" }; 
var result2 = new CustomResult(){ Id = 2, Name = "Record2", RegisteredByName = "George", ModifiedByName = null }; 
var result3 = new CustomResult(){ Id = 1, Name = "Record3", RegisteredByName = "Matt", ModifiedByName = "John" }; 

Answer 1

那麼，你需要執行left outer join爲opt有理（可爲空）域（然後訪問相關的對象屬性時，當然再檢查一下爲null）：

var results = (from r in records 
       join ru in users on r.RegisteredBy equals ru.Id 
       join mu in users on r.ModifiedBy equals mu.Id into modifiedBy 
       from mu in modifiedBy.DefaultIfEmpty() 
       select new CustomResult() 
       { 
        Id = r.Id, 
        Name = r.Name, 
        RegisteredByName = ru.Name, 
        ModifiedByName = mu != null ? mu.Name : string.Empty 
       }).ToList();