how to compare two different object lists?-CodePudding

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

CodePudding user response：

You can do this and result will be a list of ImageId

var result = ImagesByPerimeterId.Select(i => i.ImageId).
Except(ImagesWithMorePerimeters.Select(j => j.ImageId)).ToList();

CodePudding user response：

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 |