List.Except 不起作用 [英] List.Except is not working

问题描述

我尝试减去 2 个列表，如下面的代码，assignUsers 有 3 条记录，assignedUsers 有 2 行.在 Except 方法之后，我仍然得到 3 行，虽然我应该得到 1 条记录，因为 assignedUsers 中的 2 行类似于 assignUsers

I try to subtract 2 lists like below code, assignUsers has got 3 records and assignedUsers has got 2 rows. After Except method I still get 3 rows, although I should get 1 record because 2 rows in assignedUsers is similar to 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();

推荐答案

为了使 Except 方法按预期工作，AssignUserViewModel 类必须具有 GetHashCode 和 Equals 方法被正确覆盖.

In order to make Except method working as expected, the class AssignUserViewModel must have GetHashCode and Equals methods correctly overridden.

例如，如果 AssignUserViewModel 对象是由它们的 Id 唯一定义的，你应该这样定义类:

For example, if AssignUserViewModel objects are uniquely defined by their Id, you should define the class in this way:

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，如果你不能/不想改变类的实现，你可以实现一个 IEqualityComparer<> 并将它传递给 Except 方法，例如:

Otherwise, if you can't/don't want to change the class implementation, you can implement an IEqualityComparer<> and pass it to the Except method, e.g. :

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:

then your last line would become:

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

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