How to get the character index in a Silverlight TextBox in a click handler?

Question

Specifically, what I'd like is to have a user right-click in a TextBox, figure out and save the index position within the text where the right-click occurred, and then later insert some text at that position once the user makes a choice from the context menu that pops up because of the right click.

The tricky part is getting the index position based on the coordinates of the right-click.

This is in Silverlight 4.

        private int _insertPoint;
    private void userNotification_MouseRightButtonUp(object sender, MouseButtonEventArgs e)
    {
        // Move and open the context menu relative to its container.
        contextMenu.HorizontalOffset = e.GetPosition(container).X;
        contextMenu.VerticalOffset = e.GetPosition(container).Y;
        contextMenu.IsOpen = true;

        // Get the click coordinates relative to the TextBox.
        int clickX = e.GetPosition(textBox).X;
        int clickY = e.GetPosition(textBox).Y;
        _insertPoint = ?; // Here's the problem.
    }

    private void SelectFieldToInsert(object sender, MouseButtonEventArgs e)
    {
        // Close the context menu.
        contextMenu.IsOpen = false;

        var item = sender as ListBoxItem;
        textBox.Text = textBox.Text.Insert(_insertPoint, "<" + item.Content + ">");
    }

Answer 1

The TextBox.SelectionStart property on a textbox should help you. From the link:

If there is no selection, the SelectionStart value gets or sets the location of the cursor.

Answer 2

How about:

    // Get the click coordinates relative to the TextBox.
    int clickX = (int)e.GetPosition(textBox).X;
    int clickY = (int)e.GetPosition(textBox).Y;
    int startPosition = 0;
    double currentHeight = 0;
    double calculatedHeight = 0;
    int charIndex;

    TextBlock tb = new TextBlock();
    tb.Width = this.textBox.Width;
    tb.FontFamily = this.textBox.FontFamily;
    tb.FontSize = this.textBox.FontSize;
    for (charIndex = 0; charIndex < this.textBox.Text.Length; charIndex++)
    {
        tb.Text = this.textBox.Text.Substring(startPosition, charIndex - startPosition + 1);
        if (tb.ActualHeight > currentHeight)
        {
            if (currentHeight == 0)
            {
                currentHeight = tb.ActualHeight;
                calculatedHeight = currentHeight;
            }
            else
            {
                startPosition = charIndex + 1;
                charIndex++;
                tb.Text = this.textBox.Text.Substring(startPosition, charIndex - startPosition + 1);
                currentHeight = tb.ActualHeight;
                calculatedHeight += currentHeight;
            }
        }

        if (tb.ActualWidth > clickX && calculatedHeight > clickY) break;
    }

    _insertPoint = charIndex;

I won't lie and say it's elegant, but it appears to work reasonably well.