nested LINQ query - list within list

Question

I have a special issue I want to solve by using LINQ but my solution does not work yet. I condensed the structure to the minimum and hope it is easy to understand. The classes are as follows:

public class A
{
    public List<B> ListOfBs { get; set; }
}

public class B
{
    public List<C> ListOfCs { get; set; }
}

public class C
{
    public int ID { get; set; }
    public string SomeText { get; set; }
}

In my code I want to filter for specific C's and set them into a new List as property of a new instance of B (or even A). So I tried the following Linq query within a function:

    public B FilterFunction(A a, string someText)
    {
        B FilteredB = new B();
        FilteredB.ListOfCs = (from b in a.ListOfBs
                              from c in b.ListOfCs
                              where c.SomeText == someText
                              select c).ToList();
        return FilteredB;
    }

however, the returned list is empty, so I guess my LINQ query is not correct. Any suggestions? Thank you in advance Martin

To be clear, you have an A that contains a List<B> and each B contains a List<C>, and you want to create and return a single new B that contains a List<C> where each C matches a specific condition? If so then that is a job for SelectMany. I'll put together an answer to that effect. — jmcilhinney
– jmcilhinney, Commented May 10, 2021 at 5:56
Your implementation is correct. Your problem should be on data — Rodrigo Rodrigues
– Rodrigo Rodrigues, Commented May 10, 2021 at 6:18

Harald Coppoolse · Accepted Answer · 2021-05-10 08:02:01Z

So you have an object a of type A. This a has zero or more Bs, where every B has zero or more Cs. Every C has a String property SomeText.

You also have a String someText.

I want to filter for specific C's and set them into a new List as property of a new instance of B.

This is a bit ambiguous. What do you want:

I want all Cs with a value for Property SomeText that equals someText, that I can get via my a in one B. Literally your requirement says that you want one new Instance of B
No, I want to create from Every B that I can get via a, one new B. This new B has all Cs that the original B had, with a value for Property SomeText that equals someText.

Create one new instance of B

Whenever you have a sequence of items, where every item has a subsequence, and you want to regard these subsequences as one big sequence, consider to use one of the overloads of Enumerable.SelectMany.

From all Cs that are in all Bs that are in your a, you want to make one new B. Well, not exactly all Cs, only those Cs that have the correct value for property SomeText.

A a = ...
string someText = ...

var oneNewB = new B
{
    ListOfCs = a.ListOfBs.SelectMany(b => b.ListOfCs)
        .Where(c => c.SomeText == someText)
        .ToList(),
};

In words: from a, get its list of Bs. From every B, use the subsequence ListOfCs to make one big sequence of Cs. Keep only those Cs that have a value for property SomeText that equals the value of someText. Use the remaining Cs to use as ListOfCs in one new B.

Make one new B from every B in a

From every B in a.ListOfBs we make one new B. This new B contains only the Cs in the B that have a matching SomeText.

var newBs = a.ListOfBs.Select(b => new B
{
    ListOfCs = b.ListOfCs.Where(c => c.SomeText == someText).ToList(),
});

In words: From every B in a, make one new B. This new B contains only the Cs from the original B with the correct value for property SomeText.

axxel95 · Accepted Answer · 2021-05-10 05:56:32Z

1

Your code may not be optimized but it works. Did you populate your lists corectly?

B lb = new B();
lb.ListOfCs = new List<C>();
lb.ListOfCs.Add(new C { ID = 1, SomeText = "1" });
lb.ListOfCs.Add(new C { ID = 2, SomeText = "2" });
lb.ListOfCs.Add(new C { ID = 3, SomeText = "3" });
       
B lb2 = new B();
lb2.ListOfCs = new List<C>();
lb2.ListOfCs.Add(new C { ID = 10, SomeText = "10" });
lb2.ListOfCs.Add(new C { ID = 20, SomeText = "20" });
lb2.ListOfCs.Add(new C { ID = 30, SomeText = "10" });

A la = new A();
la.ListOfBs = new List<B>();
la.ListOfBs.Add(lb2);
la.ListOfBs.Add(lb);

FilterFunction(la, "10").ListOfCs.ForEach(x => Console.WriteLine(x.ID));

Result 10, 30

answered May 10, 2021 at 5:56

axxel95

414 bronze badges

Comments

Yong Shun · Accepted Answer · 2021-05-10 06:00:21Z

1

You may use .SelectMany() to flatten ListOfCs for easing the query of the C element.

.SelectMany()

Projects each element of a sequence to an IEnumerable and flattens the resulting sequences into one sequence.

public B FilterFunction(A a, string someText)
{
    B FilteredB = new B();
    FilteredB.ListOfCs = a.ListOfBs
        .SelectMany(b => b.ListOfCs)
        .Where(c => c.SomeText == someText)
        .ToList();
    
    return FilteredB;
}

Sample code & test data

answered May 10, 2021 at 6:00

Yong Shun

54.5k6 gold badges38 silver badges66 bronze badges

Comments

jmcilhinney · Accepted Answer · 2021-05-10 06:08:39Z

Using SelectMany as suggested:

class Program
{
    static void Main()
    {
        var a = new A
                {
                    ListOfBs = new List<B>
                               {
                                   new B
                                   {
                                       ListOfCs = new List<C>
                                                  {
                                                      new C {ID = 1, SomeText = "One"},
                                                      new C {ID = 2, SomeText = "Two"}
                                                  }
                                   },
                                   new B
                                   {
                                       ListOfCs = new List<C>
                                                  {
                                                      new C {ID = 3, SomeText = "Three"},
                                                      new C {ID = 4, SomeText = "Four"}
                                                  }
                                   }
                               }
                };

        var b = FilterFunction(a, "T");

        foreach (var c in b.ListOfCs)
        {
            Console.WriteLine($"{c.ID}: {c.SomeText}");
        }
    }

    public static B FilterFunction(A a, string someText)
    {
        return new B
               {
                   ListOfCs = a.ListOfBs
                               .SelectMany(b => b.ListOfCs.Where(c => c.SomeText.StartsWith(someText)))
                               .ToList()
               };
    }
}

public class A
{
    public List<B> ListOfBs { get; set; }
}

public class B
{
    public List<C> ListOfCs { get; set; }
}

public class C
{
    public int ID { get; set; }
    public string SomeText { get; set; }
}

Wow! I forgot how much quicker C# questions get answered compared to VB.NET questions.

Fabio · Accepted Answer · 2021-05-10 06:03:06Z

0

With .SelectMany you can flatten all C lists into one and filter it

public B FilterFunction(A a, string someText)
{
    return new B
    {
        ListOfCs = a.ListOfBs
            .SelectMany(bc => bc.ListOfCs)
            .Where(c => c.SomeText == someText)
            .ToList()
    }
}

answered May 10, 2021 at 6:03

Fabio

32.5k4 gold badges35 silver badges77 bronze badges

Comments

freeAll · Accepted Answer · 2021-05-10 05:56:23Z

-1

try this

public B FilterFunction(A a, string someText)
{
    B FilteredB = new B();
    FilteredB.ListOfCs = a.ListOfCs.Where(c=> c.SomeText.Equals(someText, StringComparison.InvariantCultureIgnoreCase)).ToList();
    return FilteredB;
}

answered May 10, 2021 at 5:56

freeAll

821 silver badge4 bronze badges

2 Comments

TheTanic Over a year ago

A doesn't has a ListOfC Property

freeAll Over a year ago

Oops, I did not see that, check @Fabio's answer

Collectives™ on Stack Overflow

nested LINQ query - list within list

6 Answers 6

Create one new instance of B

Make one new B from every B in a

Comments

Comments

Comments

1 Comment

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

Create one new instance of B

Make one new B from every B in a

Comments

Comments

Comments

1 Comment

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related