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

问题描述

我尝试减去以下代码中的2个列表，assignUsers有3条记录，assignedUsers有2行.在Except方法之后，我仍然得到3行，尽管我应该得到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，类AssignUserViewModel必须具有正确覆盖的GetHashCode和Equals方法.

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，则应按以下方式定义类:

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，如果您不希望更改类的实现，则可以实现IEqualityComparer<>并将其传递给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();
    }
}

然后您的最后一行将变为:

then your last line would become:

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

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