C# Linq query not working

Question

I've a Dictionary<MyEntity1, IList<MyEntity2>> and I have an object of MyEntity2 which I know it is in one (and only one) of the lists of the dictionary (I've even seen it while debugging). How ever the following query is returning null:

IDictionary<MyEntity1, IList<MyEntity2>> myDictionary = new Dictionoary<MyEntity1, IList<MyEntity2>>();
MyEntity1 myEntity1 = (from p in myDictionary
                       where p.Value.Contains(myEntity2)
                       select p.Key) as MyEntity1;

What am I doing wrong?

Please edit your post so the full definition of Dictionary<MyEntity1, IList<MyEntity2>> is visible — The Evil Greebo
– The Evil Greebo, Commented Jun 22, 2011 at 17:31
Ok, I'll edit it, but the answer posted a few seconds ago worked (I didn't get to see who posted it). — Diego
– Diego, Commented Jun 22, 2011 at 17:32

Doctor Jones · Accepted Answer · 2011-06-22 17:34:16Z

6

var myEntity1 = (from p in myDictionary
                where p.Value.Contains(myEntity2)
                select p.Key).First();

LINQ queries always return collections, even if with only one object instance

edited Jun 22, 2011 at 17:34

Doctor Jones

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answered Jun 22, 2011 at 17:32

Joao

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2 Comments

Diego Over a year ago

Will this return null if no object found?

Omer Bokhari Over a year ago

@Diego: no, it will not return null... it will throw an exception if not found. use.FirstOrDefault() if you want a null instead.

Henk Holterman · Accepted Answer · 2011-06-22 17:33:47Z

1

You should use something like

   MyEntity1 myEntity1 = (from p in myDictionary
                   where p.Value.Contains(myEntity2)
                   select p.Key).FirstOrDefault();

The query will return an IEnumerable<MyEntity1> and casting that to MyEntity1 will always return null. The cast is invalid and as will return null.

answered Jun 22, 2011 at 17:33

Henk Holterman

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Comments

Stuart · Accepted Answer · 2011-06-22 17:34:16Z

1

Your problem is that the Select query returns an IEnumerable - which you can't cast to MyEntity1.

Try adding FirstOrDefault() - then you won't need the cast.

IDictionary<MyEntity1, IList<MyEntity2>> myDictionary = new Dictionary<MyEntity1, IList<MyEntity2>>();
var myEntity1 = (from p in myDictionary
                   where p.Value.Contains(myEntity2)
                   select p.Key).FirstOrDefault();

answered Jun 22, 2011 at 17:34

Stuart

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