Question

I am trying to create a TreeView nested structure with the use of self referencing table fields. Here is a simple example:

Category 1 
      Product 1 
          Toy 1 
          Toy 2 
      Product 2 
          Toy 3 
          Toy 4 

more categories..

The database table has a single table called “Category”. The ParentCategoryId points to the Category which is the parent. So, for the Category 1 the ParentCategoryId is null since it is parent. For Product 1 the ParentCategoryId is that of the Category 1 id and for Toy 1 the ParentCategoryId is that for the Product 1 id.

I am using the following code but it does not generate the TreeView (ASP.NET) successfully.

 public void BuildTree(List<Category> categories, TreeNode treeNode)
    {
        if (treeNode == null) return;

        TreeNode tnAdd = null;
        var categoryId = Guid.NewGuid();

        foreach (var category in categories)
        {
            if (category.IsBaseCategory)
            {
                tnAdd = new TreeNode();
                tnAdd.Text = category.Description;

                BuildTree((from c in categories
                           where c.ParentCategoryId == category.CategoryId
                           select c).ToList<Category>(), tnAdd);
            }
            else
            {
                tnAdd = new TreeNode();
                tnAdd.Text = category.Description;

                BuildTree((from c in categories
                           where c.ParentCategoryId == category.CategoryId
                           select c).ToList<Category>(), tnAdd);
            }

            if (tnAdd != null)
                treeNode.ChildNodes.Add(tnAdd);              
        }
    }

Does this require recursion!

and here is the result I get:

  80W 
  40W 
  40W 
  Light Bulbs 

   Flourecent 
   Incedecent 

  60W 
  80W 
  60W 
  Flourecent 

   40W 
   80W 
   60W 

  Incedecent 

   80W 
   40W 
   60W 

Answer 1

What isn’t successful?
If its because you see nothing … I don’t see where you’re ever adding the root node to the actual tree control. tnAdd needs to be added to the tree control somewhere.

If it’s because your not getting everything you expect: Unless you already have recursion going on somewhere and don’t realize it, I can’t see how the above code is ever going to get to the toy level. You say “base”, then “child” in the above code which covers two levels. You have three levels in your sample data, so at some point you need to account for adding the toys. You can write it recursively if you need to have n levels. If you only have three levels, you can just repeat yourself.

—– UPDATES FOR OP UPDATES

Looking at your code, what you have is this:

   for each category {
    if it is base
       add its children
    else if it is not base
       add its children

    add it to the tree
   }

This means every item is hit in the first foreach and added to the tree, rather than per level.
what you want is

for each category{ 
    if it is base
       add base's children

       for each child [
          add child's children
          add child to the tree
       ]

       add base the tree
}

Something closer to this (I don’t have time to test right now, sorry) should come close to working

 public BuildTreeTop(List<Category> categories, TreeNode treeNode)
 {
    BuildTree((from c in categories
                           where c.IsBaseCategory == true
                           select c).ToList<Category>(), categories, tnAdd);
 }

 public void BuildTree(List<Category> currentLevel, List<Category> allCategories, TreeNode treeNode)
    {
        if (treeNode == null) return;

        TreeNode tnAdd = null;
        var categoryId = Guid.NewGuid();

        foreach (var category in currentLevel)
        {
            tnAdd = new TreeNode();
            tnAdd.Text = category.Description;

            BuildTree((from c in allCategories
                        where c.ParentCategoryId == category.CategoryId
                        select c).ToList<Category>(), allCategories, tnAdd);


            if (tnAdd != null)
                treeNode.ChildNodes.Add(tnAdd);              
        }
    }