Empty Popup window for datagridcell

I'm using a Popup (System.Windows.FrameworkElement.Popup) to display a note to the user if e.g. he exceeds the maximum string length in a text box.

I'm using the PlacementTarget property to assign a frameworkElement to make sure that the Popup is displayed near the corresponding control and not near to the mouse pointer.

Everything is working fine for a normal textBox, but using a popup in 开发者_JS百科a datagrid poses a problem.

Using the popup with PlacementTarget = theDataGrid is working but the popup is displayed at the left upper corner of the datagrid (with Placement = PlacementMode.Left) but I want to display the popup near the cell that triggers the display of the popup.

Now I am using the following code to determine the corresponding DataGridCell:

static DataGridCell TryToFindGridCell(DataGrid grid, DataGridCellInfo cellInfo)
{
DataGridCell result = null;
DataGridRow row = (DataGridRow)grid.ItemContainerGenerator.ContainerFromItem(cellInfo.Item);
if (row != null)
{
    int columnIndex = grid.Columns.IndexOf(cellInfo.Column);
    if (columnIndex > -1)
    {
        DataGridCellsPresenter presenter = VisualTreeHelperMethods.GetVisualChild<DataGridCellsPresenter>(row);
        result = presenter.ItemContainerGenerator.ContainerFromIndex(columnIndex) as DataGridCell;
    }
}

return result;
}

which seems to work fine, but using the DataGridCell (which is a FrameworkElement in contrast to DataGridCellInfo) has the effects, that

• the popup is displayed at the correct location, near the DataGridCell

• but is empty

Can anybody help or explain why the popup is empty?

For now I stick to displaying the popup with PlacementTarget = theDataGrid which is however a bit unsatisfactory.