C# compare a list of IDs to each ID in a list of objects using LINQ

Question

I'm attempting to compare list of ints to a list of objects. to see if one of the IDs match the a key in each object. If it does then return true, else false.

for example:

List<int> ints = new List<int> () { 1, 2, 3, 4, 5};
List<someObjectType> objects = new List<someObjectType> () { {1, 'one'}, {6, 'six'}, {45, 'forty-five'} };

(x => x.objects.any(a => ints.contains(x.id)));

However I don't know how to compare a list of ints to only one property on an object, I only know how to compare whole arrays to each other.

Answer 1

Are you looking for something like that?

  List<int> ints = new List<int> () { 1, 2, 3, 4, 5};

  // For better performance when "ints" is long
  HashSet<int> ids = new HashSet<int>(ints);

  List<someObjectType> objects = new List<someObjectType> () { 
    {1, "one"}, {6, "six"}, {45, "forty-five"} };

  // if any is matched?
  boolean any = objects
    .Any(item => ids.Contains(item.id));

  // All that match
  var objectsThatMatch = objects
    .Where(item => ids.Contains(item.id));

Answer 2

Your pseudo-code is pretty much there (though with Where instead of Any )...

var objectsWithIds = objects.Where(o => ints.Contains(o.Id));

Which makes me suspect this isn't what you are after...

Another way is using Intersect , but you need to translate them both into an IEnumerable<> of the same type.

var intersectionOfIds = objects.Select(_ => _.Id).Intersect(ints);

But this only gets you a list of Ids that are in both lists, not the objects themselves, which you would then need to go and find again.

Answer 3

Instead of object , you could use the keyword dynamic , but an even better solution would be to use some static type (if possible).

You can use dynamic like a dictionary, the added link will provide you with some great examples.

Answer 4

我离得很近

(x => x.objects.any(a => ints.contains(a.id)));