C# LINQ recursive query to show parent child relation

Question

I have a example which showing parent child relation in hierarchical way but one minor issue facing. i have a property called hierarchy which will show 0000+ID for parent row which has parent id is zero and for all child rows it will have Parent hierarchy plus child id. for this issue i am not being able to do it. so please have look at my code and tell me how to fix this area.

private void button1_Click(object sender, EventArgs e)
{
    List<Comment> categories = new List<Comment>()
    {
        new Comment () { Id = 1, Text = "Item 1", ParentId = 0},
        new Comment() { Id = 2, Text = "Item 2", ParentId = 1 },
        new Comment() { Id = 3, Text = "Item 3", ParentId = 2 },
        new Comment() { Id = 4, Text = "Item 1.1", ParentId = 1 },
        new Comment() { Id = 5, Text = "Item 3.1", ParentId = 3 },
        new Comment() { Id = 6, Text = "Item 1.1.1", ParentId = 4 },
        new Comment() { Id = 7, Text = "Item 2.1", ParentId = 2 }
    };

    List<Comment> hierarchy = new List<Comment>();
    hierarchy = categories
                    .Where(c => c.ParentId == 0)
                    .Select(c => new Comment()
                    {
                        Id = c.Id,
                        Text = c.Text,
                        ParentId = c.ParentId,
                        hierarchy="0000" + c.Id,
                        Children = GetChildren(categories, c.Id)
                    })
                    .ToList();

    HieararchyWalk(hierarchy);
}

        public List<Comment> GetChildren(List<Comment> comments, int parentId)
        {
            return comments
                    .Where(c => c.ParentId == parentId)
                    .Select(c => new Comment
                    {
                        Id = c.Id,
                        Text = c.Text,
                        ParentId = c.ParentId,
                        hierarchy = "0000" +comments.Where(a => a.Id == parentId).FirstOrDefault().Id + ".0000" + c.Id,
                        Children = GetChildren(comments, c.Id)
                    })
                    .ToList();
        }

public void HieararchyWalk(List<Comment> hierarchy)
{
    if (hierarchy != null)
    {
        foreach (var item in hierarchy)
        {
            //Console.WriteLine(string.Format("{0} {1}", item.Id, item.Text));
            textBox1.Text += item.Id + " " + item.Text+ "Hierarchy-"+ item.hierarchy + System.Environment.NewLine;
            HieararchyWalk(item.Children);
        }
    }
}

public class Comment
{
    public int Id { get; set; }
    public int ParentId { get; set; }
    public string hierarchy { get; set; }
    public string Text { get; set; }
    public List<Comment> Children { get; set; }
}

When calling HieararchyWalk() then hierarchy property not showing value right way.

hierarchy not showing in chain wise. parent hierarchy -> child hierarchy -> again child hierarchy this way it should be shown.

Parent hierarchy is not getting carry forward for nested child....where the problem lies ? please help me to fix this problem. Thanks

Answer 1

There is no problem with your Linq statement, but the Text property in the example does not seem to correspond to the actual structure.

I just modified its Text property, and then to make the demonstration clearer, I added it to the TreeView instead of the Textbox.

        private void button1_Click(object sender, EventArgs e)  
        {  
            List<Comment> categories = new List<Comment>()  
                                         {  
                                             new Comment () { Id = 1, Text = "Item 1", ParentId = 0},  
                                             new Comment() { Id = 2, Text = "Item 1.1", ParentId = 1 },  
                                             new Comment() { Id = 3, Text = "Item 1.1.1", ParentId = 2 },  
                                             new Comment() { Id = 4, Text = "Item 1.2", ParentId = 1 },  
                                             new Comment() { Id = 5, Text = "Item 1.1.1.1", ParentId = 3 },  
                                             new Comment() { Id = 6, Text = "Item 1.2.1", ParentId = 4 },  
                                             new Comment() { Id = 7, Text = "Item 1.1.2", ParentId = 2 }  
                                         };  
  
            List<Comment> hierarchy = new List<Comment>();  
            hierarchy = categories  
                            .Where(c => c.ParentId == 0)  
                            .Select(c => new Comment()  
                            {  
                                Id = c.Id,  
                                Text = c.Text,  
                                ParentId = c.ParentId,  
                                hierarchy = "0000" + c.Id,  
                                Children = GetChildren(categories, c.Id)  
                            })  
                            .ToList();  
            TreeNode rootNode = treeView1.Nodes.Add("Root");  
            HieararchyWalk(hierarchy, rootNode);  
        }  
        public void HieararchyWalk(List<Comment> hierarchy, TreeNode treeNode)  
        {  
            if (hierarchy != null)  
            {  
                foreach (var item in hierarchy)  
                {  
                    TreeNode newTreeNode = treeNode.Nodes.Add(item.Id+ " " + item.Text + " Hierarchy-" + item.hierarchy);  
                    if (item.Children.Count != 0)  
                    {  
                        HieararchyWalk(item.Children, newTreeNode);  
                    }  
                }  
            }  
        }  

Did I understand what you mean correctly?

Update(5/17):

Please try the following code:

        public List<Comment> GetChildren(List<Comment> comments, int parentId)  
        {  
            return comments  
                    .Where(c => c.ParentId == parentId)  
                    .Select(c => new Comment  
                    {  
                        Id = c.Id,  
                        Text = c.Text,  
                        ParentId = c.ParentId,  
                        hierarchy = GetHiera(comments,c,parentId),  
                        Children = GetChildren(comments, c.Id)  
                    })  
                    .ToList();  
        }  
        public string GetHiera(List<Comment> comments, Comment comment, int parentId)   
        {  
            string hierarchy = "0000"+ comment.Id+" ";  
            Comment parentComm = comments.Where(a => a.Id == parentId).FirstOrDefault();  
            if (parentComm.ParentId!=0)  
            {  
                hierarchy = GetHiera(comments, parentComm, parentComm.ParentId)+hierarchy;  
            }  
            else  
            {  
                hierarchy ="0000"+ parentId +" "+ hierarchy+" " ;  
            }  
            return hierarchy;  
        }  

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Answer 2

Hello,
I got the same solution in .Net Core WEB API. Please check the link: https://youtu.be/SzyReVDLB2M