How to find all occurance of a particular entity from nested List using Linq

Question

5.00/5 (1 vote)

See more:

I have a nested class like following:

C#

public class Group
{

    public int Id { get; set; }

    public string Description { get; set; }

    public int ParentId { get; set; }

    public List<Group> Groups { get; set; }

}

I have a List of class Group like

C#

List<Group> gp=new List<Group>();

My requirement is to find all the Id values of list gp using Linq.I tried with Selectmany(), but its returning me only the first level of hirarchy.

Posted 21-Sep-15 8:41am

Alivia Banerjee

Updated 21-Sep-15 8:46am

v2

Add a Solution

Comments

Maciej Los 21-Sep-15 15:47pm

What exactly have you tried? What is input data?
Nested classes are classes where the class declaration is fully contained within the class declaration of another class. Your class definition does not look like nested class.

2 solutions

Solution 2

Here's an alternative approach that I use which doesn't require storing the entire hierarchy in a list, and isn't tied to a particular data structure:

C#

private static IEnumerable<TResult> SelectRecursiveIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TSource>> getChildren, Func<TSource, int, TResult> selector)
{
    var stack = new Stack<IEnumerator<TSource>>();

    try
    {
        stack.Push(source.GetEnumerator());
        while (0 != stack.Count)
        {
            var iter = stack.Peek();
            if (iter.MoveNext())
            {
                var current = iter.Current;
                yield return selector(current, stack.Count - 1);

                var children = getChildren(current);
                if (children != null) stack.Push(children.GetEnumerator());
            }
            else
            {
                iter.Dispose();
                stack.Pop();
            }
        }
    }
    finally
    {
        while (0 != stack.Count)
        {
            stack.Pop().Dispose();
        }
    }
}

public static IEnumerable<TResult> SelectRecursive<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TSource>> getChildren, Func<TSource, int, TResult> selector)
{
    if (source == null) throw new ArgumentNullException("source");
    if (selector == null) throw new ArgumentNullException("selector");
    if (getChildren == null) return source.Select(s => selector(s, 0));
    return SelectRecursiveIterator(source, getChildren, selector);
}

public static IEnumerable<T> SelectRecursive<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> getChildren)
{
    if (source == null) throw new ArgumentNullException("source");
    if (getChildren == null) return source;
    return SelectRecursiveIterator(source, getChildren, (s, _) => s);
}

To get the list of all IDs, you would then use something like this:

C#

List<int> allIds = groups.SelectRecursive(g => g.Groups).Select(g => g.Id).ToList();

Posted 22-Sep-15 7:44am

Richard Deeming

Comments

Matt T Heffron 22-Sep-15 15:57pm

Nice!
I wasn't willing to think that hard! :-)
+5

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Matt T Heffron · Accepted Answer · 2015-09-21T12:39:00

You're going to have to write recursion somewhere outside of the Linq query. Linq does not do recursion on its own.
Something like:

C#

public static List<Group> Flatten(List<Group> groups)
{
  List<Group> result = new List<Group>();
  if (groups != null)
  {
    // a bit of a hack to get around the self referential lambda issue.
    // (And, Yes, it must be two separate statements, and the null initialization is required)
    Action<Group> recurser = null;
    recurser = g => {
      if (g != null)
      {
        g.Groups.ForEach(recurser);
        result.Add(g);
      }
    };
    groups.ForEach(recurser);
  }
  return result;
}

(This is a pre-order, depth-first, traversal.)

Now you can:

C#

List<int> AllIds = Flatten(RootGroupList).Select(g => g.Id).ToList();

Edit:
An additional note: This doesn't check that the Groups property isn't null. A relatively safe way to ensure this would be to change:

C#

public class Group
{ 
    public int Id { get; set; }
    public string Description { get; set; }
    public int ParentId { get; set; }
    private readonly List<Group> _Groups = new List<Group>();
    public List<Group> Groups { get { return _Groups; } }
}

However, this prevents wholesale replacing the Groups property. Its contents must be manipulated with .Clear(), .Add(), etc.