how to compare two different object lists?

Question

I have these two lists:

List<image> ImagesByPerimeterId
List<PerimeterTile> ImagesWithMorePerimeters

The context is the following: I want to remove images that contain the id found in the ImagesWithMorePerimeters list from the ImagesByPerimeterId list. The ImagesWithMorePerimeters list has an imageId attribute to compare with the first one.

I have implemented this logic, and it works very well:

foreach(var i in ImagesByPerimeterId) 
{
  foreach(var j in ImagesWithMorePerimeters) 
  {
    if (i.Id == j.ImageId) 
    {
      ImagesByPerimeterId.Remove(i);
    }
  }
}

but I'm looking for a simpler way to compare these lists. Any suggestions?

I tried to use list.Except(), but as the lists are different objects, that did not make it

Does this answer your question? [C# comparing two large lists of items by a specific property](https://stackoverflow.com/questions/55138128/c-sharp-comparing-two-large-lists-of-items-by-a-specific-property) — BurnsBA, Nov 17 '22 at 14:25
There are many different ways to do this. One way is to extract ids you do/don't want, and then filter the collection, e.g., `.Where(x => ids.Contains(x.ImageId))`. Or you can make a custom comparer to pass to `Except`. — BurnsBA, Nov 17 '22 at 14:27
Define "simpler way"? Your code works pretty well and in fact *is* pretty simple from my point of view. — MakePeaceGreatAgain, Nov 17 '22 at 14:30
@Selvin That's what I'd expected as well, however OP states it *does* work. — MakePeaceGreatAgain, Nov 17 '22 at 14:33
Hehe [...](https://dotnetfiddle.net/6OToCJ) so you think that the real OP problem is this exception? — Selvin, Nov 17 '22 at 14:34

score 0 · Accepted Answer · answered Nov 17 '22 at 15:07

You are trying to modify the list while iterating through it, that will cause an exception when the method Remove() is called:

InvalidOperationException: Collection was modified; enumeration operation may not execute.

If you want to do it using iteration do this:

for (var i2 = 0; i2 < ImagesWithMorePerimeters.Count; i2++)
{
    for (var i1 = ImagesByPerimeterId.Count - 1; i1 >= 0; i1--)
    {
        if (ImagesWithMorePerimeters[i2].ImageId == ImagesByPerimeterId[i1].Id)
            ImagesByPerimeterId.RemoveAt(i1);
    }
}

If you want to make it shorter you can do this using Linq:

ImagesByPerimeterId.RemoveAll(image => ImagesWithMorePerimeters.Any(imageMore => image.Id == imageMore.ImageId));

Linq version will be about 5 times slower, here are the benchmark:

|         Method |      Mean |
|--------------- |----------:|
| UsingIteration |  2.314 us |
|      UsingLinq | 10.348 us |

Both your versions should be `O(n*m)`, It should be possible to do better by using some kind of lookup table for comparisons, like a HashSet. — JonasH, Nov 17 '22 at 15:22

how to compare two different object lists?

1 Answers1