List.Except 不起作用

我尝试减去 2 个列表，如下代码所示，assignUsers已获得 3 条记录assignedUsers有 2 行。后Except方法我仍然得到 3 行，尽管我应该得到 1 条记录，因为 2 行assignedUsers类似于assignUsers

 var users = accountApp.GetUsersByAccountId(context.GetUserData().AccountId);
 List<AssignUserViewModel> assignUsers = Mapper.Map<List<AssignUserViewModel>>(users).ToList();
 var mailUsers = mailApp.GetMailAssignedByMailId(id).Select(m => new { m.UserId, m.User.Name }).ToList();
 List<AssignUserViewModel> assignedUsers = mailUsers.Select(Mapper.DynamicMap<AssignUserViewModel>).ToList();
 assignUsers = assignUsers.Except(assignedUsers).ToList();

为了要做Except方法按预期工作，类AssignUserViewModel一定有GetHashCode and Equals方法被正确覆盖。

例如，如果AssignUserViewModel对象由它们的唯一定义Id，你应该这样定义类：

class AssignUserViewModel
{
    // other methods...


    public override int GetHashCode()
    {
        return this.Id.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        if (!(obj is AssignUserViewModel))
            throw new ArgumentException("obj is not an AssignUserViewModel");
        var usr = obj as AssignUserViewModel;
        if (usr == null)
            return false;
        return this.Id.Equals(usr.Id);
    }
}

否则，如果您不能/不想更改类实现，您可以实现IEqualityComparer<>并将其传递给Except方法，例如:

class AssignUserViewModelEqualityComparer : IEqualityComparer<AssignUserViewModel>
{
    public bool Equals(AssignUserViewModel x, AssignUserViewModel y)
    {
        if (object.ReferenceEquals(x, y))
            return true;
        if(x == null || y == null)
            return false;
        return x.Id.Equals(y.Id);
    }

    public int GetHashCode(AssignUserViewModel obj)
    {
        return obj.Id.GetHashCode();
    }
}

那么你的最后一行将变成：

assignUsers = assignUsers.Except(assignedUsers, new AssignUserViewModelEqualityComparer()).ToList();