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Introduction  

Having two generic lists (List<MyClass>), how do I remove from List1 all the instances of List2, matching a property?

This is the setup. I have a class:  

public class MyClass
{
    public int MyValue { get; set; }
    public int MyOtherValue { get; set; }
}

And two lists:

List<MyClass> list1 = new List<MyClass>();
list1.Add(new MyClass() { MyValue = 1, MyOtherValue = 10 });
list1.Add(new MyClass() { MyValue = 2, MyOtherValue = 20 });
list1.Add(new MyClass() { MyValue = 3, MyOtherValue = 30 });
list1.Add(new MyClass() { MyValue = 4, MyOtherValue = 40 });

List<MyClass> list2 = new List<MyClass>();
list2.Add(new MyClass() { MyValue = 2, MyOtherValue = 50 });
list2.Add(new MyClass() { MyValue = 3, MyOtherValue = 60 });

I want to remove all list2 instances from list1 matching the MyValue property.
 

Solution 

list2.ForEach(l2 => { list1.RemoveAll(l1 => l1.MyValue == l2.MyValue); });

(from l1 in list1 join l2 in list2 on l1.MyValue equals l2.MyValue
 select l1).ToList().ForEach(i => list1.Remove(i));

MyValue: 1 MyOtherValue: 10
MyValue: 4 MyOtherValue: 40

Performance

List1 size: 1.000.000
{
     List2 size: 10
     Method 1: 103 ms
     Method 2: 325 ms

     List2 size: 100
     Method 1: 5534 ms
     Method 2: 682 ms
}

List1 size: 1.000
{
     List2 size: 100
     Method 1: 1 ms
     Method 2: 11 ms

     List2 size: 900
     Method 1: 32 ms
     Method 2: 12 ms
}

And it gets all bad when both lists are large:

List1 size: 100.000
{
     List2 size: 90.000
     Method 1: 297834 ms
     Method 2: 7555 ms
}

Conclusion 

Use Method2 unless you are sure that the list that you want to remove is very small compared to the main list (<50 elements).

History

2009-11-06: First version

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My vote of 2 Paulo Zemek 1:02 10 Nov '09   Very basic article. Also, using HashSets, Dictionaries and KeyedLists is probably a better approach. Sign In·View Thread·PermaLink Re: My vote of 2 Henrik Sterndorff Jessen 6:27 11 Nov '09   That is very true - and performance is way better. But does the dictionary/keyed list approach not require that you know in advance what property you want to search by? Of cause you can always build two new dictionaries and filter using them. But I think that makes less readable code (unless you hide it away in an utility method). Anyways... I was looking for an easy one-liner. Do you have a better way to do it (sans the dictionary approach)? Sign In·View Thread·PermaLink

Last Visit: 11:15 21 Nov '09     Last Update: 11:15 21 Nov '09 1