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I have two collections

List<CustomClass1> items1 List<CustomClass2> items2 CustomClass1 has a property KEY CustomClass2 has a property KEY

i want to keep only those entries in items1 which have a matching key in items2. How can this be achieved through LINQ?

thanks

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  • @six: I think linq-to-objects should still apply here. This specifically is using LINQ to objects and methods used here might not apply to other providers. Commented Jul 21, 2011 at 22:33
  • @Jeff: I figured it was implied given he has two List<T>s. I won't object to it being re-tagged as such. Commented Jul 21, 2011 at 22:46

4 Answers 4

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var res = items1.Join(items2, item1 => item1.Key, item2 => item2.Key, (item1, item2) => item1);
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3 Comments

When they ask for LINQ, actually use LINQ itself please! :\ Not just the methods.
@Megrdad This is perfectly valid Linq code. It's Lambda syntax instead of query syntax thats all.
@asawyer: But AFAIK "LINQ" means "language-integrated query", and from what I see, there's nothing different about this language than normal C#.
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var res = items1.Where(a=> items2.Any(c=>c.Key == a.Key));
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thanks.. will this modify the original collection .. i mean items1.
no, it doesn't. No standard (as in: from the .net framework) LINQ operator modifies the original collection.
@Jeff Mercado Could you briefly explain your performance concerns here? I'm just curious.
@asawyer: This right off the bat would become a O(n*m) operation. For every item in items1, we're performing a (linear) search in items2. Using a join on the other hand could be (and probably is) optimized to do some hashing. So I would avoid this approach if I had the choice.
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var q = from i1 in items1 join i2 in items2 on i1.Key equals i2.Key select i1;
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@Mehrdad It's interesting that these are all pretty much the same, this should compile to something like Femaref's answer.
Yes, this translates to my query.
@Mehrdad: It's all LINQ. Using the query syntax doesn't make it more LINQy than not. LINQ isn't just about the syntax, but also the libraries that back it up.
@Mehrdad You may have a slight misconception on what is and is not Linq.
Nope Mehrdad. This is the query syntax, mine is the method chain syntax. LINQ is both.
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You could always use the Intersect operator-

var result = item1.Intersect(item2);

If necessary the overload allows an equity comparer, although if your items are from the same context it shouldn't be necessary

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It only works if they are collections of the same type. They are not in the question however.
And you are completely correct.. missed that the collections were different types. Thanks!

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