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.
var list = new List
Solution for objects (maybe easier than horaces solution):
If your list contains objects, rather than scalars, it is that simple, by removing by one selected property of the objects:
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. If your list contains an object, rather than a scalar, you need to do a bit more work.
I tried this over and over with Remove() and RemoveAt() and all sorts of things and none of them worked correctly. I couldn't even get a Contains() to work correctly. Never matched anything. I was stumped until I got the suspicion that maybe it could not match up the item correctly.
When I realized this, I refactored the item class to implement IEquatable, and then it started working.
Here is my solution:
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.
See: http://msdn.microsoft.com/en-us/library/bhkz42b3.aspx
list = list.Except(list2).ToList();
You can do this via RemoveAll using Contains:
list.RemoveAll( item => !list2.Contains(item));
Alternatively, if you just want the intersection, using Enumerable.Intersect would be more efficient:
list = list.Intersect(list2).ToList();
The difference is, in the latter case, you will not get duplicate entries. 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}
.