[英]Remove items from list 1 not in list 2
I am learning to write lambda expressions , and I need help on how to remove all elements from a list which are not in another list. 我正在学习编写lambda表达式 ,我需要有关如何从列表中删除不在另一个列表中的所有元素的帮助。
var list = new List<int> {1, 2, 2, 4, 5};
var list2 = new List<int> { 4, 5 };
// Remove all list items not in List2
// new List Should contain {4,5}
// The lambda expression is the Predicate.
list.RemoveAll(item => item. /*solution expression here*/ );
// Display results.
foreach (int i in list)
{
Console.WriteLine(i);
}
You can do this via RemoveAll using Contains: 您可以使用Contains通过RemoveAll执行此操作:
list.RemoveAll( item => !list2.Contains(item));
Alternatively, if you just want the intersection, using Enumerable.Intersect would be more efficient: 或者,如果您只想要交集,使用Enumerable.Intersect会更有效:
list = list.Intersect(list2).ToList();
The difference is, in the latter case, you will not get duplicate entries. 不同的是,在后一种情况下,您将不会获得重复的条目。 For example, if list2
contained 2, in the first case, you'd get {2,2,4,5}
, in the second, you'd get {2,4,5}
. 例如,如果list2
包含2,则在第一种情况下,您将获得{2,2,4,5}
,在第二种情况下,您将获得{2,4,5}
。
Solution for objects (maybe easier than horaces solution): 对象的解决方案 (可能比horaces解决方案更容易):
If your list contains objects, rather than scalars, it is that simple, by removing by one selected property of the objects: 如果您的列表包含对象而不是标量,那么通过删除对象的一个选定属性就可以了:
var a = allActivePatientContracts.RemoveAll(x => !allPatients.Select(y => y.Id).Contains(x.PatientId));
This question has been marked as answered, but there is a catch. 这个问题已被标记为已回答,但有一个问题。 If your list contains an object, rather than a scalar, you need to do a bit more work. 如果列表包含对象而不是标量,则需要做更多的工作。
I tried this over and over with Remove() and RemoveAt() and all sorts of things and none of them worked correctly. 我一遍又一遍地尝试使用Remove()和RemoveAt()以及各种各样的东西,但没有一个能正常工作。 I couldn't even get a Contains() to work correctly. 我甚至无法让Contains()正常工作。 Never matched anything. 从来没有匹配过。 I was stumped until I got the suspicion that maybe it could not match up the item correctly. 我很难过,直到我怀疑它可能无法正确匹配项目。
When I realized this, I refactored the item class to implement IEquatable, and then it started working. 当我意识到这一点时,我重构了item类来实现IEquatable,然后它开始工作了。
Here is my solution: 这是我的解决方案:
class GenericLookupE : IEquatable<GenericLookupE>
{
public string ID { get; set; }
public bool Equals( GenericLookupE other )
{
if ( this.ID == other.ID ) return true;
return false;
}
}
After I did this, the above RemoveAll() answer by Reed Copsey worked perfectly for me. 在我这样做之后,Reed Copsey的上述RemoveAll()回答对我来说非常合适。
See: http://msdn.microsoft.com/en-us/library/bhkz42b3.aspx 请参阅: http : //msdn.microsoft.com/en-us/library/bhkz42b3.aspx
list = list.Except(list2).ToList();
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