How can I intersect two different .NET lists by a single property?

Question

I'm trying to filter my first list of Foo's based on some values in a second list of Baa's.

For example.

Here's an example I put up on .NET Fiddle ...

var foos = new List<Foo>
{
    new Foo { Name = "Leia" },
    new Foo { Name = "Han Solo" },
    new Foo { Name = "Chewbacca" },
    new Foo { Name = "Luke" },
};

    var baas = new List<Baa>
{
    new Baa { Alias = "aaaaa" },
    new Baa { Alias = "bbbb" },
    new Baa { Alias = "Leia" },
    new Baa { Alias = "Luke" }
};

// Expected output:
// List<Foo> results = Foo { "Leia" } and Foo { "Luke" };

See how I'm asking for: Filter the first list (by Name) by the second lists Alias property.

and that will return a List of Foo with 2 results in it?

Any clues?

If your collections contain a lot of items, it might be wise to use `HashSet` instead of `List`. Checking if an item is contained in a `HashSet` is a O(1) operation, which can greatly improve the speed of computing the intersection. There is a dedicated method for this in the `HashSet` class, see: http://msdn.microsoft.com/en-us/library/vstudio/bb918911%28v=vs.90%29.aspx — Benlitz, Apr 21 '14 at 13:12
*VERY* good point @Benlitz ! In my scenario, each list will be a few items only. But the answer(s) really help me for future dev where I might have to deal with list intersections. Would love to see some example code using the `Intersects` keyword if the two collections were `HashSet`'s. — Pure.Krome, Apr 21 '14 at 13:15
@Pure.Krome You can't use `Intersect` because the lists are of two different types. — D Stanley, Apr 21 '14 at 13:29
Yep I misread the question, however you can still do it if you use a `HashSet — Benlitz, Apr 21 '14 at 14:00

score 10 · Answer 1 · edited Apr 21 '14 at 13:21

10

You can use Any on the list of baas:

foos.Where(f => baas.Any(b => b.Alias == f.Name));

Or use a join (cleaner in query syntax):

var query = 
    from f in foos
    join b in baas on f.Name equals b.Alias
    select f;

Here's the code in a full working .NET Fiddle.

edited Apr 21 '14 at 13:21

joce

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answered Apr 21 '14 at 13:06

D Stanley

149,601
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score 5 · Answer 2 · edited May 23 '17 at 12:30

5

You could use Join:

var results = foos.Join(baas, f => f.Name, b => b.Alias, (f, b) => f);

How can I intersect two different .NET lists by a single property?

2 Answers2