Can't get UIAutomationElement BoundingRectangle in win7 by suriyel

Hi,
I use IRawElementProviderFragment for testing,but I can't get UIAutomationElement's BoundingRectangle in win7.When I try to get it,the process stop working.
The following code is how I get the BoundingRectangle in my tableCell using the interface of IRawElementProviderFragment:


        /// <summary>
        /// Gets the bounding rectangle of this element.
        /// </summary>
        /// <value></value>
        /// <returns>The bounding rectangle, in screen coordinates.</returns>
        System.Windows.Rect IRawElementProviderFragment.BoundingRectangle
        {
            get
            {
                var cellPoint = _grid.GetCellDisplayRectangle(_cell.ColumnIndex, _cell.RowIndex, true);
                var formPoint = new Point(cellPoint.X, cellPoint.Y);
                _grid.Invoke(new Action(() => formPoint = _grid.PointToScreen(formPoint)));
                var topPoint = new CursorPoint() { X = formPoint.X, Y = formPoint.Y };
                var widthPoint = new CursorPoint() {X = cellPoint.Width + formPoint.X, Y = formPoint.Y};
                var heightPoint = new CursorPoint() {X = formPoint.X, Y = formPoint.Y + cellPoint.Height};
                _grid.Invoke(new Action(() =>
                    {
                        try
                        {
                            LogicalToPhysicalPoint(_grid.Handle, ref topPoint);
                            LogicalToPhysicalPoint(_grid.Handle, ref widthPoint);
                            LogicalToPhysicalPoint(_grid.Handle, ref heightPoint);
                        }
                        catch (Exception)
                        {
                            ;
                        }
                    }));
                var width = widthPoint.X - topPoint.X;
                var height = heightPoint.Y - topPoint.Y;
                return new System.Windows.Rect(topPoint.X, topPoint.Y, width, height);
            }
        }
The following code is how I get the BoundingRectangle:
System.Windows.Rect boundingRect1;
object boundingRectNoDefault =
    autoElement.GetCurrentPropertyValue(AutomationElement.BoundingRectangleProperty, true);
if (boundingRectNoDefault == AutomationElement.NotSupported)
{
return null;
}
else
{
    boundingRect1 = (System.Windows.Rect)boundingRectNoDefault;
}



And I notice the article about the dpi.But it still dosen't work.
                                                                                                 Best wishes.