How to iterate hierarchical data and output the hierarchy using C#

Question

given the following class ...

public class Category {
  public string Name {get;set;}
  public Category ParentCategory {get;set;}
}

What the most efficient way to output the following from a collection (IList<Category>) of Category objects?

+ Parent Category
++ Sub Category
++ Sub Category 2
+ Parent Category 2
++ Sub ...
++ Sub ..
++ Sub ....

EDIT: Perhaps the real question should be, how should I represent this model in the database and retrieve it using NHibernate?

Answer 1

You may wish to consider reversing your relationship. If a node can get to its parent but not vice versa, you have to have all the leaf nodes in order to print out the full tree. Compare this to the situation where you have each node know about its children - then you only need the root node.

Answer 2

recently i read about Hierarchical Linq Queries, check out http://weblogs.asp.net/okloeten/archive/2006/07/09/Hierarchical-Linq-Queries.aspx

but I tend to aggree with Jon Skeet.

Answer 3

A small recursive function can do it for you.

static void recurseCategories(ref List<Category> cl, Category start, int level)
{
  foreach (Category child in cl)
  {
    if (child.ParentCategory == start)
    {
      Console.WriteLine(new String(' ', level) + child.Name);
      recurseCategories(ref cl, child, level + 1);
    }
  }
}

My assumptions were:

• You've got an List of Category. (Of course all Category objects you want to print must be in that list. I thought that was self-evident, but seemingly it was not.)
• The root category has a parent of null. The initial function call should therefore be recurseCategories(ref myCategoryList, null, 0).
• No orphaned elements exist in your list. Some error handling code should be added.
• Output order will be coherent to whatever order the list is iterated, so apart from the hierarchy it's more or less coincidental.

Answer 4

@Tomalok, when I run your code, with this data structure, I get two nodes:

class Program
{
    static void Main(string[] args)
    {
        var foo = new List<Category>();

        // Example structure from Question
        var fail1 = new Category() { Name = "test1" };
        var fail2 = new Category() { Name = "test2", ParentCategory = fail1 };
        var fail3 = new Category() { Name = "test3", ParentCategory = fail1 };
        var fail4 = new Category() { Name = "test4"};
        var fail5 = new Category() { Name = "test5", ParentCategory = fail4 };
        var fail6 = new Category() { Name = "test6", ParentCategory = fail4 };
        var fail7 = new Category() { Name = "test7", ParentCategory = fail4 };

        foo.Add(fail1);
        foo.Add(fail4);

        recurseCategories(ref foo, null, 0);

    }

    static void recurseCategories(ref List<Category> cl, Category start, int level)
    {
        foreach (Category child in cl)
        {
            if (child.ParentCategory == start)
            {
                Console.WriteLine(new String(' ', level) + child.Name);
                recurseCategories(ref cl, child, level + 1);
            }
        }
    }

    public class Category
    {
        public string Name { get; set; }
        public Category ParentCategory { get; set; }
    }

}

Output:

test1
test4

Output should be:

test1
   test 2
   test 3
test 4
   test 5
   test 6
   test 7

This is because there is no relationship between child nodes that share the same parent.