Wpf Textbox Spellcheck issue with Custom Dictionary - wpf

Question

I have got spell checking issues with the custom dictionary in a Wpf application in that it does not recognize I∆n.
If I add a behavior to try and deal with I∆n, DoSpellCheck() works when you type or paste text into the Textbox but does not work when textBox_Loaded(object sender, RoutedEventArgs e) is called.
Any help would appreciated.
public class TextBoxExtendedSpellCheckBehavior : Behavior
{
private static Key[] _nonInputKeys = new Key[] { Key.Left, Key.Up, Key.Right, Key.Down, Key.Tab, Key.Delete, Key.Back, Key.Escape, Key.Enter };
protected override void OnAttached()
{
if (AssociatedObject is TextBox)
{
TextBox textBox = AssociatedObject as TextBox;
textBox.PreviewKeyUp += OnTextBox_PreviewKeyUp;
textBox.LostFocus += OnTextBox_LostFocus;
textBox.TextChanged += OnTextChanged;
textBox.Loaded += textBox_Loaded;
}
base.OnAttached();
}
void textBox_Loaded(object sender, RoutedEventArgs e)
{
TextBox textBox = sender as TextBox;
if (textBox != null)
{
DoSpellCheck(textBox);
}
}
protected override void OnDetaching()
{
if (AssociatedObject is TextBox)
{
TextBox textBox = AssociatedObject as TextBox;
textBox.PreviewKeyUp -= OnTextBox_PreviewKeyUp;
textBox.LostFocus -= OnTextBox_LostFocus;
textBox.Loaded -= textBox_Loaded;
}
base.OnDetaching();
}
private static void OnTextChanged(object sender, TextChangedEventArgs e)
{
if (sender is TextBox)
{
TextBox textBox = sender as TextBox;
if (textBox != null && textBox.IsLoaded == true)
{
DoSpellCheck(textBox);
}
}
}
private bool IsInputKey(Key key)
{
return !_nonInputKeys.Contains(key);
}
private void OnTextBox_PreviewKeyUp(object sender, KeyEventArgs e)
{
if (sender is TextBox && IsInputKey(e.Key))
{
TextBox textBox = sender as TextBox;
DoSpellCheck(textBox);
}
}
private void OnTextBox_LostFocus(object sender, RoutedEventArgs e)
{
if (sender is TextBox)
{
TextBox textBox = sender as TextBox;
DoSpellCheck(textBox);
}
}
public static void DoSpellCheck(TextBox textBox)
{
const string iDelta = "I∆n";
int index = textBox.Text.IndexOf(iDelta);
if (index > 0)
{
int iStart = textBox.GetSpellingErrorStart(index);
if (iStart > -1)
{
SpellingError spellingError = textBox.GetSpellingError(iStart);
if (spellingError != null)
{
spellingError.IgnoreAll();
}
}
}
}
}