無法使用LINQ或lambda查詢從列表中選擇許多項

Question

我有靜態類中的對象列表。 像這樣：

public static class ClassOfItems
{
     private static List<Item> ListOfItems = new List<Item>();
     public static List<Item> GetAll()
     {
        return ListOfItems;
     }
}
public class Item
{
     public int id{get; set;}
     public string name{get;set:}
}

我從XML文件初始化項目。 直到那一刻，一切都OK。 例如，我的靜態類中有兩個項目：

 item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example): 
 1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method. 

item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example):
1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method.

 item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example): 
 1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method. 

item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example):
1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method.

 item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example): 
 1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method. 

item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example):
1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method.

item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example):
1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method.

 item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example): 
 1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method. 

---

item1: id=1 name="item1" item2: id=2 name="item2" Now I want to select items where id value is more then 0 (for example):
1 way: var items=from i in ClassOfItems.GetAll() where i.id>0 select i; 2 way: var items=ClassOfItems.GetAll().Where(i => i.id>0); Both of them not working :( I just noticed that when I use methods with returns single objects it's working. For example: var items=ClassOfItems.GetAll().First(i => i==1);//it's working var items=ClassOfItems.GetAll().Where(i=> i==1);//not working ;( haha solved sorry guys :D just needed to use ToList() method.

 var items=(ClassOfItems.GetAll().Where(i => i.id>0)).ToList(); 

Answer 1

Select()轉換對象。 因此，在最后一個Select語句中，您將返回bool

如果期望一個結果，則使用First(i=> i.id==1)或FirstOrDefault(i=> i.id==1)如果您期望一個結果，則使用Where(i=> i.id==1)期待多種結果。

Answer 2

如果查看文檔，則將看到Select和Where方法返回IEnumerable<TResult> ，即使您的序列只有一個元素也沒關系。

實際上，您的Where方法按預期工作。如果您在Where之后看到foreach循環遍歷您的列表，那我是正確的。使用yield return的可枚舉擴展方法，這就是為什么在調試器中看不到列表的原因。 WhereListIterator. 您可以查看此文檔以獲取有關yield關鍵字的更多詳細信息。

您可以通過使用foreach語句或LINQ查詢來使用迭代器方法。 foreach循環的每次迭代都調用iterator方法。 在迭代器方法中到達yield return語句時，將返回expression，並保留代碼中的當前位置。 下次調用迭代器函數時，將從該位置重新開始執行。

但是，如果要獲取列表，則在以下位置使用ToList方法：

var items=ClassOfItems.GetAll().Where(i=> i==1).ToList();