How to create a Add function for a record database?

In my program, I need to put an add function to insert categories. I'm using a treeView to show the data.

How can I model this class in a database?

The user insert the level, and the program must insert that category in that level. But I'm getting tired. Because it needs to check if the others levels exist (IE: The treeView is empty and I want to add 2.1, so it's an error).

Sometimes, you may add an already level setted and so that must be forbidden.

I need a little of help in my code. It's finished, but I want to improve it or fixed errors (if so).

Here is the code:

    private voi开发者_StackOverflow中文版d AddButton_Click(object sender, RoutedEventArgs e)
    {
        NorthwindDataContext cd = new NorthwindDataContext();

        int[] levels = LevelTextBox.Text.ToIntArray('.');
        string newName = NameTextBox.Text;

        int[] parentLevels = new int[levels.Length - 1];
        Array.Copy(levels, parentLevels, parentLevels.Length);
        Objective current = GetNode(levels);
        Objective parent = GetNode(parentLevels);

        if (current != null)
        {
            MessageBox.Show("Level already exists");
            return;
        }
        else if (parent == null && parentLevels.Length != 0)
        {
            MessageBox.Show("Parent level doesn't exist");
            return;
        }

        var newObjective = new Objective();
        newObjective.Name = newName;
        newObjective.Level = levels.Last();
        newObjective.Parent_ObjectiveID = parent == null ? null : (int?)parent.ObjectiveID;

        cd.Objective.InsertOnSubmit(newObjective);
        cd.SubmitChanges();

        MessageBox.Show("The new objective has added successfully");
        NameTextBox.Clear();
        LoadObjectives();
    }

    public Objective GetNode(params int[] indexes)
    {
        return GetNode(null, 0, indexes);
    }

    public Objective GetNode(int? parentid, int level, params int[] indexes)
    {
        NorthwindDataContext cd = new NorthwindDataContext();
        Objective item = null;

        if (indexes.Length == 0)
            return null;

        if (parentid == null)
        {
            item = (from p in cd.Objective
                    where p.Level == indexes[level] && p.Parent_ObjectiveID == null
                    select p).SingleOrDefault();

        }
        else
        {
            item = (from p in cd.Objective
                    where p.Level == indexes[level] && p.Parent_ObjectiveID == parentid
                    select p).SingleOrDefault();
        }

        if (item == null)
            return null;

        if (++level < indexes.Length)
            item = GetNode(item.ObjectiveID, level, indexes);

        return item;
    }

Why don't you let the user select a parent node to which the new category should be added? You could let them just click a parent node and then add a new node to it. No checks involved. I know that this does not answer your direct question, but your current approach is hard on you and your users.