# How can I improve this code about categories and subcategories?

This is the code that I'm using. I'm working on LINQ TO SQL and I'm using this model in my program:

How can I model this class in a database

The person must have to enter the level, for example 1.2, 1.1.4, etc. The program must check if the level is available or busy. The function ToIntArray is an extension, consists of splitting the text and converting the resulting string[] to int[].

The first condition, as you can see, is an inconsistency which I have at this post:

Problems with nullable types in a LINQ function

private void AddButton_Click(object sender, RoutedEventArgs e)
{
NorthwindDataContext cd = new NorthwindDataContext();

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

Objective currentObjective = null;
int? identity = null;

for (int i = 0; i < levels.Length - 1; i++ )
{
int currentRank = levels[i];

if (identity == null)
{
currentObjective = (from p in cd.Objective
where p.Level == currentRank && p.Parent_ObjectiveID == null
select p).SingleOrDefault();
}
else
{
currentObjective = (from p in cd.Objective
where p.Level == currentRank && p.Parent_ObjectiveID == identity
select p).SingleOrDefault();
}

if (currentObjective == null)
{
MessageBox.Show("Levels don't exist");
return;
}
else
{
identity = currentObjective.ObjectiveID;
}
}

if (currentObjective != null)
{
if (levels.Last() == currentObjective.Level)
{
return;
}
}
else
{
var aux = (from p in cd.Objective
where p.Parent_ObjectiveID == null && p.Level == levels.Last()
select p).SingleOrDefault();

if (aux != null)
{
return;
}
}

var newObjective = new Objective();
newObjective.Name = NameTextBox.Text;
newObjective.Level = levels.Last();
newObjective.Parent_ObjectiveID = currentObjective == null ? null : (int?)currentObjective.ObjectiveID ;

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

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

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1. You should move this code from the AddButton_Click to some business logic class.
2. Maybe you should add some validation of LevelTextBox.Text, before using its value.
3. You should split this code into different methods.

You have three responsibilities here:

1. Find out if the level exists at all.
2. Find out if the specified level already exists.
3. Create a new level.

The fact that all three actions use the same data doesn't mean you need to put them in one method.

4. Maybe I get it wrong somewhere, but I can't understand how this code checks if the level already exists. I can't see how the code will ever get into the MessageBox.Show("Level already exists"); in both places.

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