Creating A TreeView Nested Structure Using Self Referencing Table?

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;[code].....