linq物體相等以及如何正確覆蓋它

Question

爲什麼var excludes = users.Except(matches);不排除matches？

如果我想讓相等比較器僅使用ID，那麼正確的方法是什麼？例如，將不勝感激。

public class User 
{ 
    public int ID { get; set; } 
    public string Name { get; set; } 

    public override string ToString() 
    { 
     return ID.ToString() + ":" + Name; 
    } 
} 

private static void LinqTest2() 
{ 
    IEnumerable<User> users = new List<User> 
    { 
     new User {ID = 1, Name = "Jack"}, 
     new User {ID = 2, Name = "Tom"}, 
     new User {ID = 3, Name = "Jim"}, 
     new User {ID = 4, Name = "Joe"}, 
     new User {ID = 5, Name = "James"}, 
     new User {ID = 6, Name = "Matt"}, 
     new User {ID = 7, Name = "Jon"}, 
     new User {ID = 8, Name = "Jill"} 
    }.OfType<User>(); 

    IEnumerable<User> localList = new List<User> 
    { 
     new User {ID = 4, Name = "Joe"}, 
     new User {ID = 5, Name = "James"} 
    }.OfType<User>(); 


    //After creating the two list 
    var matches = from u in users 
        join lu in localList 
        on u.ID equals lu.ID 
        select lu; 
    Console.WriteLine("--------------MATCHES----------------"); 
    foreach (var item in matches) 
    { 
     Console.WriteLine(item.ToString()); 
    } 

    Console.WriteLine("--------------EXCLUDES----------------"); 
    var excludes = users.Except(matches); 
    foreach (var item in excludes) 
    { 
     Console.WriteLine(item.ToString()); 
    }    
} 

Answer 1

sealed class CompareUsersById : IEqualityComparer<User> 
    { 
     public bool Equals(User x, User y) 
     { 
      if(x == null) 
       return y == null; 
      else if(y == null) 
       return false; 
      else 
       return x.ID == y.ID; 
     } 

     public int GetHashCode(User obj) 
     { 
      return obj.ID; 
     } 
    } 

然後

var excludes = users.Except(matches, new CompareUsersById()); 

Answer 2

您的用戶類沒有重載Equals和GetHashCode因此使用的Equals默認實現。在這種情況下，這意味着它會比較參考。您創建了兩個具有相同值的用戶對象，但由於它們是不同對象，所以它們會比較不相等。

重寫Equals的替代方法是使用overload of Except that takes an IEqualityComparer。