linq对象相等以及如何正确覆盖它

Question

为什么var excludes = users.Except(matches);不排除matches？

如果我希望相等比较器只使用ID？,那么正确的方法是什么？例子将不胜感激.

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;
        }
    }

然后

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