Question: I Want to display POPUP specific position of RichTextBox in WPF . i have understood that there is a way to get the same in winforms RichTextBox with following lines of code.

```
Point point = richTextBox1.GetPositionFromCharIndex(richTextBox1.SelectionStart);

```

Answer: 
I suppose it depends on when and what you are popping up. An example from MSDN shows how to position a `ContextMenu` with the location of the selected text within a `RichTextBox` control.
[How to: Position a Custom Context Menu in a RichTextBox](http://msdn.microsoft.com/en-us/library/vstudio/ms752316%28v=vs.90%29.aspx)
The interesting bit would be the code below:

```
TextPointer position = rtb.Selection.End;

if (position == null) return;

Rect positionRect = position.GetCharacterRect(LogicalDirection.Forward);
contextMenu.HorizontalOffset = positionRect.X;
contextMenu.VerticalOffset = positionRect.Y;

```

This gets the relative position of the selection. If you are popping up a form you will need to translate that into a Window position.

This is a bit of code I used to test loading a popup window over the selected text in a `RichTextBox`. This also takes into account multiple monitors.

```
TextPointer tp = txtEditor.Selection.End;
if (tp == null) return;
Rect charRect = tp.GetCharacterRect(LogicalDirection.Forward);
Point winPoint = txtEditor.PointToScreen(charRect.TopRight);
Popup p = new Popup();
p.Left = winPoint.X;
p.Top = winPoint.Y;
p.Show();

```

**UPDATE:**
I did some additional research and found a MSDN [Popup Placement Behavior](http://msdn.microsoft.com/en-us/library/bb613596%28VS.100%29.aspx) article that is likely what you are looking for as far as `Popup` behavior. You can use the code I provided above with the selection or caret position of the RichTextBox to then determine the ultimate positioning of the `Popup`. I hope that helps.
```
    static void tb_PreviewKeyDown(object sender, System.Windows.Input.KeyEventArgs e)
    {

        if (e.KeyStates == ((e.KeyStates ^ System.Windows.Input.KeyStates.Down)^System.Windows.Input.KeyStates.Down))
        {
            if (e.Key == System.Windows.Input.Key.OemPeriod)
            {

                TextBox tb = (TextBox)sender;

                Rect r = tb.GetRectFromCharacterIndex(tb.CaretIndex, true);
                Point p = tb.TransformToAncestor(tb).Transform(new Point(r.X, r.Y + 10));
                p = tb.PointToScreen(p);

                Rect rect = new Rect(p.X, p.Y, 0, 0);
                Grid g = (Grid)Application.Current.MainWindow.Content;
                System.Windows.Controls.Primitives.Popup popup = new System.Windows.Controls.Primitives.Popup();
                popup.SetValue(System.Windows.Controls.Primitives.Popup.PlacementRectangleProperty, rect);

                popup.IsOpen = true;
                g.Children.Add(popup);}}}

```