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.

Are you working against an anonymous type? You won't be able to access the properties without dynamic or reflection for runtime binding, or var for compile-time binding. In a List<anon>, var is not usable. Your use of an anonymous type here seems contrived. — Adam Houldsworth
– Adam Houldsworth, Commented Apr 30, 2015 at 15:13
Why are you using List<object> instead of a Dictionary, or at least a List<KeyValuePair<int, string>>? — Callum Bradbury
– Callum Bradbury, Commented Apr 30, 2015 at 15:14
I agree with @CallumBradbury. The List<object> should be Dictionary<int,string> or List<KeyValuePair<int,string>>. — Dustin Kingen
– Dustin Kingen, Commented Apr 30, 2015 at 15:14

Dmitrii Bychenko · Accepted Answer · 2017-04-03 06:58:56Z

6

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));

edited Apr 3, 2017 at 6:58

answered Apr 30, 2015 at 15:23

Dmitrii Bychenko

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3 Comments

user3754124 Over a year ago

with a little work this ended up being the solution (p => p.object != null && p.object.Any(o => ids.Contains(o.id)));

Hogan Over a year ago

This looks to me like it would not compile -- 'one' is not a valid C# constant.

Dmitrii Bychenko Over a year ago

@Hogan: you're quite right it sould be "one", "six" etc. I've just copied the code from the question

Adam Houldsworth · Accepted Answer · 2015-04-30 15:24:31Z

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.

edited Apr 30, 2015 at 15:24

answered Apr 30, 2015 at 15:19

Adam Houldsworth

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Comments

Sándor Mátyás Márton · Accepted Answer · 2015-04-30 15:15:50Z

0

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.

answered Apr 30, 2015 at 15:15

Sándor Mátyás Márton

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1 Comment

user3754124 Over a year ago

sorry - fixed. Object was just a placeholder for an actual object type.

user3754124 · Accepted Answer · 2015-04-30 15:32:56Z

0

I was close

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

answered Apr 30, 2015 at 15:32

user3754124

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