Show ToolTip on RichTextBox

I ha开发者_如何学JAVAve a RichTextBox on a WPF Window. Now, I want to show a ToolTip when the user move the mouse over the RichTextBox. The Content of the RichTextBox should depend on the Text which is under the mouse pointer. For this I should get the position of the char, on which the mouse shows to.

Best Regards, Thomas

In the following example the tooltip will show the next character where the caret is.

Xaml:

<Window x:Class="Test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
  <RichTextBox ToolTipOpening="rtb_ToolTipOpening" ToolTip="" />
</Window>

Code-behind:

void rtb_ToolTipOpening(object sender, ToolTipEventArgs e)
{
  RichTextBox rtb = sender as RichTextBox;

  if (rtb == null)
    return;

  TextPointer position = rtb.GetPositionFromPoint(Mouse.GetPosition(rtb), false);
  if (position == null)
    return;

  int offset = rtb.Document.ContentStart.GetOffsetToPosition(position);

  position = rtb.Document.ContentStart.GetPositionAtOffset(offset);
  if (position == null)
    return;

  string text = position.GetTextInRun(LogicalDirection.Forward);

  rtb.ToolTip = !string.IsNullOrEmpty(text) ? text.Substring(0, 1) : string.Empty;
}