C# linq expression in lambda with contains

Question

I am trying to make use of the 'contains' to simulate the old SQL 'where id in (1,2,3,4)' way of filtering a query.

However I have some difficulties in using it where my id's are in a deeper level.

Code:

 public class Category
    {
        public long Id { get; set; }
        public string Name { get; set; }
    }

    public class Characteristica
    {
        public Category Category { get; set; }
        public int Id { get; set; }
        public string Value { get; set; }
    }

    public class Person
    {
        public string Name { get; set; }
        public List<Characteristica> Appearance { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var persons = new List<Person>
            {
                new Person { Name = "Person A", Appearance = new List<Characteristica> { new Characteristica { Id = 22 }, new Characteristica { Id = 5 }, new Characteristica { Id = 12 } }},
                new Person { Name = "Person B", Appearance = new List<Characteristica> { new Characteristica { Id = 1 }, new Characteristica { Id = 6 }, new Characteristica { Id = 11 } }},
                new Person { Name = "Person C", Appearance = new List<Characteristica> { new Characteristica { Id = 2 }, new Characteristica { Id = 8 }, new Characteristica { Id = 13 } }},
                new Person { Name = "Person D", Appearance = new List<Characteristica> { new Characteristica { Id = 2 }, new Characteristica { Id = 5 }, new Characteristica { Id = 10 } }},
                new Person { Name = "Person E", Appearance = new List<Characteristica> { new Characteristica { Id = 1 }, new Characteristica { Id = 8 }, new Characteristica { Id = 10 } }},
                new Person { Name = "Person F", Appearance = new List<Characteristica> { new Characteristica { Id = 1 }, new Characteristica { Id = 6 }, new Characteristica { Id = 23 } }},
            };

            var listOfSearchedIds = new List<int> { 22, 23 };
            var selected = persons.Select(p => p.Appearance.Where(a => listOfSearchedIds.Contains(a.Id))).ToList();
        }
    }

Now I am trying to get 'Person A' and 'Person F' out from my collection by using the contains feauture. However I cannot see what I am doing wrong here.

Can someone shed some light on what I am doing wrong? I have tried different versions of my lambda and this is the closes I can get, but I am getting all 6 items out from my expression.

What does happen? do you get an error? Do you get the wrong information? I had assumed you were running against a database but your example does not look like you are. Can you put it on complify.net? — George Mauer
– George Mauer, Commented Feb 20, 2014 at 21:19
In line with the answer from Markus maybe use a HashSet rather than a List for Characteristica. — paparazzo
– paparazzo, Commented Feb 20, 2014 at 21:39
No, I did not get an error. I was just expecting to get a list of the person that I need. My example is with objects, but in real code I am retrieving data from a database using EF 5. — codingjoe
– codingjoe, Commented Feb 20, 2014 at 22:23

Selman Genç · Accepted Answer · 2014-02-20 21:31:21Z

34

Your way is correct but you should use Where instead of Select

 var selected = persons.Where(p => p.Appearance
                .Where(a => listOfSearchedIds.Contains(a.Id))
                .Any()).ToList();

And you need to use Any to check whether the returning sequence from p.Appearance.Where contains any element.Or you can use Any directly and make it shorter:

var selected = persons.Where(p => p.Appearance
                .Any(a => listOfSearchedIds.Contains(a.Id))
                .ToList();

answered Feb 20, 2014 at 21:31

Selman Genç

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1 Comment

Latin Warrior Over a year ago

I tested both methods in LINQPad 5 and they both yielded the same SQL in the end. DECLARE p0 Int = 2 DECLARE p1 Int = 3 SELECT [t0].[Id], [t0].[Name], [t0].[Description], [t0].[ModifiedBy], [t0].[ModifiedDate], [t0].[CreatedBy], [t0].[CreatedDate], [t0].[RowVersion], [t0].[TriggerTypeGuid] FROM [TriggerType] AS [t0] WHERE EXISTS(SELECT NULL AS [EMPTY] FROM [TriggerClass] AS [t1] WHERE ([t1].[TriggerTypeId] IN (p0, p1)) AND ([t1].[TriggerTypeId] = [t0].[Id])) So Selman is correct in pointing out that either method works but the second one is shorter, but maybe less explicit.

Markus · Accepted Answer · 2014-02-20 21:36:17Z

3

Try the following:

var listOfSearchedIds = new List<int> { 22, 23 };
var selected = persons
        .Where(p => listOfSearchedIds
                        .Intersect(p.Appearance
                             .Select(a => a.Id)).Any()).ToList();

By using Intersect, you compare two lists and return the items that are contained in both of them. Intersect uses a HashSet internally and therefore is a very performant way two find the intersection of two sets. Any() returns true if there is at least one item in the resulting list.

edited Feb 20, 2014 at 21:36

answered Feb 20, 2014 at 21:31

Markus

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1 Comment

Jim Wooley Over a year ago

Intersect will work with LINQ to objects, but if you want this against a database (EF/L2S), you need to go with Contains.

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