Re: Linq - Remove Object From Collection - C# Discussion Boards

8-Dec-11 7:19

I have a list of these models:

public class NodeModel : ViewModelBase
{
    public Guid Id { get; set; }
    public string Caption { get; set; }
    public string Description { get; set; }
    public bool IsActive { get; set; }
    public int Sequence { get; set; }
    public ImageSource Image { get; set; }
    public bool IsExpanded { get; set; }
    public string FileName { get; set; }
    public NodeType Type { get; set; }
    public List<NodeModel> Children { get; set; }

    public NodeModel()
    {
        Children = new List<NodeModel>();
        FileName = string.Empty;
    }
}

The list is contained in a collection called Folders. I want to remove an object from the collection, so I'm using:

Folders.Remove(f => f.Id == model.Id);

But it's not removing it, What's wrong here?

Everything makes sense in someone's mind

fjdiewornncalwe8-Dec-11 7:37

fjdiewornncalwe

8-Dec-11 7:37

I know I'm stating the bloody obvious, but have you placed a breakpoint on the Folders.Remove statement to make sure you have a proper model.Id? You can also convert the statement into a foreach temporarily so that you have more debugging visiblity. Once the problem is solved, I would certainly return the statement into the linq syntax, though.

I wasn't, now I am, then I won't be anymore.

Dan Mos8-Dec-11 7:46

8-Dec-11 7:46

Remove doesn't take a predicate as far as I know.

Either use .RemoveAll(f => f.Id == model.Id) or use Remove(specificInstanceOfNode)
Smile | :)

All the best,

Dan

Kevin Marois8-Dec-11 7:49

8-Dec-11 7:49

Ok, so now I have this:

NodeModel folder = (from f in Folders
                    where f.Id == model.Id
                    select f).FirstOrDefault();
Folders.Remove(folder);

The query is returning null. It's not finding the folder.

It appears that the query is only looking at the first level and not drilling down into the collection.

Everything makes sense in someone's mind

Dan Mos8-Dec-11 7:56

8-Dec-11 7:56

Yes. It sure doesn't. I completely missed the inner list.

So you want to remove from the ChildNode inner list or from the main/upper list or from both?

All the best,

Dan

Kevin Marois8-Dec-11 7:57

8-Dec-11 7:57

I want to find i anywhere in the list and remove it.

Everything makes sense in someone's mind

Dan Mos8-Dec-11 8:22

8-Dec-11 8:22

Well this is more complicated then it looks. Cause you don't know upfront how
many nested levels you have.

As each node has a list of childnodes and then each node in the nested childNodes list could have an
even deeper nested level list of childNodes ...

The only thing that comes to mind wright now is a brute force step by step actions:

1)ForEach item in the upper level list check to see if it has the id and remove it if so.
2) Else loop through the inner collection if Count>0 and remove from there else loop through it's inner collection if Count>0
and so on.

Unless you do know that there are only 2 levels.

All the best,

Dan

Kevin Marois8-Dec-11 8:24

8-Dec-11 8:24

There can be any levels. I could write a recursive method, but I think that will cause problems modifying the collection.

Everything makes sense in someone's mind

Dan Mos8-Dec-11 9:18

8-Dec-11 9:18

True, a faster approach would be to use a boolean filed like obsolete/deleted/visible.

Then instead of deleting/altering the collection you would just set the value to true(no list and/or memory reallocation).

When populating UI or whatever, based on the boolean field you'll show it or not.

All the best,

Dan

two compile time errors with your for loop based code: Re: Linq - Remove Object From Collection

BobJanova8-Dec-11 22:44

BobJanova

8-Dec-11 22:44

Only if you use foreach ... it's a list so you can use integer indices. Not thread-safe but self-modification-safe:

void Remove<T>(IList<T> list, T node) where T:INested<T> {
 if(list == null) return;  // in case you use ChildNodes=null to specify no children
 for(int i = list.Count; i >= 0; i--){
  if(list[i] == node) list.RemoveAt(i);
  else Remove(list[i].ChildNodes, node);
 }
}

where

interface INested<T> { IList<T> ChildNodes; }

If this is the only place you need something like this, you don't have to generic it, but it is a general algorithm so you might want to put it in a utility library and have your class implement that interface.

BillWoodruff9-Dec-11 9:16

BillWoodruff

9-Dec-11 9:16

Definition of the INested<T> interface will not compile since ChildNodes is a field. So, for testing your code, I'll be starting with this:

XML

interface INested<T>
{
    IList<T> ChildNodes { get; set; }
}

In your code, where you compare:

if (list[i] == node) list.RemoveAt(i);

the compiler throws an error: " Operator '==' cannot be applied to operands of type 'T' and 'T' "

best, Bill

"For no man lives in the external truth among salts and acids, but in the warm, phantasmagoric chamber of his brain, with the painted windows and the storied wall." Robert Louis Stevenson

BillWoodruff9-Dec-11 9:17

BillWoodruff

9-Dec-11 9:17

I'm going to be studying this code carefully today, since it appears to me a very valuable alternative to using foreach iteration, or using Linq to kind of create a copy of the recursive data structure from which you can modify the original structure without the "modification" error.

My guess is that this (once compile-time errors are fixed) would make a great Tip/Trick for that section of CP.

thanks, Bill

"For no man lives in the external truth among salts and acids, but in the warm, phantasmagoric chamber of his brain, with the painted windows and the storied wall." Robert Louis Stevenson

modified 9-Dec-11 15:49pm.

Mirko198012-Dec-11 2:53

Mirko1980

12-Dec-11 2:53

I've further elaborated the idea above, using an extension method:

// Base interfaces for a node
public interface INode<T> 
    where T : INode<T> // A child is the same class as the parent
{
    Guid Id { get; set; }
    List<T> Children { get; set; }
}

// Static class containing Extension Methods
public static class NodeExtensions
{
    // Recursively removes a node by its id
    public static void Remove<T>(this INode<T> node, Guid id)
        where T : INode<T>
    {
        // List that will contains the nodes to be removed
        // Can't remove nodes directly in the foreach loop
        // (will throw "Collection was modified; enumeration operation may not execute.")
        List<T> toRemove = new List<T>();

        if (node.Children == null)
            return; // No childrens, exit

        foreach (var child in node.Children)
        {
            if (child.Id == id)
                toRemove.Add(child); // The node has to be removed
            else
                child.Remove(id); // Recursion here
        }

        // Removes all found nodes
        foreach (var child in toRemove)
            node.Children.Remove(child);
    }
}