Question

I have a categories table which is set up to allow an infinite number of sub category levels. I would like to mimic the following:

It should be clarified that sub categories can have sub categories. E.g. Parent cat -> level 1 -> level 2 -> level 3 etc.

My categories table has two columns, CategoryName and ParentID.

This list box will be used when assigning the correct category to a product.

How can I write this?

Edit

In response to thedugas I had to modify your answer to work with my situation. I found some errors that needed to be fixed, but below is a final, working solution.

protected void Page_Load(object sender, EventArgs e)
    {
        using (DataClasses1DataContext db = new DataClasses1DataContext())
        {

        var c = db.Categories.Select(x => x);

        List<Category> categories = new List<Category>();
        foreach (var n in c)
        {
            categories.Add(new Category()
            {
                categoryID = n.categoryID,
                title = n.title,
                parentID = n.parentID,
                isVisible = n.isVisible
            });
        }
        List<string> xx = new List<string>();

        foreach (Category cat in categories)
        {
            BuildCatString(string.Empty, cat, categories, xx);
        }

        ListBox1.DataSource = xx;
        ListBox1.DataBind();

    }
}

private void BuildCatString(string prefix, Category cat, IEnumerable<Category> categories, List<string> xx)
{
    if (cat.parentID == 0)
    {
        xx.Add(cat.title);
        prefix = cat.title;
    }

    var children = categories.Where(x => x.parentID == cat.categoryID);
    if (children.Count() == 0)
    {
        return;
    }

    foreach (Category child in children)
    {
        if(prefix.Any())
        {
        xx.Add(prefix + "/" + child.title);
        BuildCatString(prefix + "/" + child.title,
            child, categories, xx);
        }
    }

}

Here is the almost finished work:

Answer 1

Nick asked me in a comment to another question how this sort of problem might be solved using LINQ to Objects without using any recursion. Easily done.

Let’s suppose that we have a Dictionary<Id, Category> that maps ids to categories. Each category has three fields: Id, ParentId and Name. Let’s presume that ParentId can be null, to mark those categories that are “top level”.

The desired output is a sequence of strings where each string is the “fully-qualified” name of the category.

The solution is straightforward. We begin by defining a helper method:

public static IEnumerable<Category> CategoryAndParents(this Dictionary<Id, Category> map, Id id)
{
    Id current = id;
    while(current != null)
    {
        Category category = map[current];
        yield return category;
        current = category.ParentId;
    }
}

And this helper method:

public static string FullName(this Dictionary<Id, Category> map, Id id)
{
    return map.CategoryAndParents(id)
              .Aggregate("", (string name, Category cat) =>
                cat.Name + (name == "" ? "" : @"/") + name);
}

Or, if you prefer avoiding the potentially inefficient naive string concatenation:

public static string FullName(this Dictionary<Id, Category> map, Id id)
{
    return string.Join(@"/", map.CategoryAndParents(id)
                                .Select(cat=>cat.Name)
                                .Reverse());
}

And now the query is straightforward:

fullNames = from id in map.Keys
            select map.FullName(id);
listBox.DataSource = fullNames.ToList();

No recursion necessary.