Implement “drag and fill” functionality in WPF dat

We are developing an app that utilises a data grid. Most of the grid's functionality will be fairly standard stuff - styling cells, in-cell editing, sorting, etc. However one requirement is to provide "drag and fill" functionality, like Excel. This is where you select one or more cells inside a row, which then get a thick border drawn around them. The border has a small square on its bottom-right corner, and when you drag this down Excel copies the selected cell values to the rows below.

This sounds like a frightening thing to try and implement using the .Net WPF data grid (and no commercial data grid component provides this feature either). Has anyone developed a similar feature, or suggest possible approaches to accomplishing this? Plain old copy & paste is not an option!

Here are some tips to draw a thick line around your selected cells in a datagrid:

Subscribe to the datagrid's SelectedCellsChanged event.
In the event handler, get the adorner layer of the datagrid and add a custom adorner to it if the number of selected cells > 0 and if there is no (cached) adornerlayer yet. Pass the datagrid via the adorner's constructor. If the number of selected cells == 0 and the cached adornerlayer != null, remove the adorner from the adorner layer. At the end of the event handler, call adornerLayer.Update(datagrid)

Use the following CustomAdorner class:

public class DataGridSelectionAdorner : Adorner {
    private readonly DataGrid datagrid;
    private readonly SolidColorBrush backgroundBrush = new SolidColorBrush(Color.FromArgb(30, 0, 0, 0));
    readonly Pen pen = new Pen(new SolidColorBrush(Colors.Black), 1);
    private readonly Dictionary<DataGridCellInfo, int[]> cellInfoToTableRowAndColumn = new Dictionary<DataGridCellInfo, int[]>();

    public DataGridSelectionAdorner(UIElement adornedElement)
        : base(adornedElement) {
        datagrid = (DataGrid)adornedElement;
        pen.DashStyle = new DashStyle(new[] { 3.0, 3.0 }, 0);
        IsHitTestVisible = false;
    }

    protected override void OnRender(DrawingContext drawingContext) {
        ItemContainerGenerator generator = datagrid.ItemContainerGenerator;
        IEnumerable<int> rows =
                datagrid.SelectedCells.Select(c =>
                    generator.IndexFromContainer(
                        generator.ContainerFromItem(c.Item)
                    )
                );
        IEnumerable<int> columns = datagrid.SelectedCells.Select(
            c => c.Column.DisplayIndex
        );
        int minRow = rows.Min();
        int maxRow = rows.Max();
        int minColumn = columns.Min();
        int maxColumn = columns.Max();

        foreach (var cell in datagrid.SelectedCells) {
            int row = generator.IndexFromContainer(generator.ContainerFromItem(cell.Item));
            int column = cell.Column.DisplayIndex;
            cellInfoToTableRowAndColumn[cell] = new[] { row, column };
        }

        var topLeft = cellInfoToTableRowAndColumn.First(c => c.Value[0] == minRow && c.Value[1] == minColumn).Key;
        var bottomRight = cellInfoToTableRowAndColumn.First(c => c.Value[0] == maxRow && c.Value[1] == maxColumn).Key;

        var topLeftCell = GetDataGridCell(topLeft);
        var bottomRightCell = GetDataGridCell(bottomRight);

        const double marginX = 4.5;
        const double marginY = 3.5;
        Point topLeftPoint = topLeftCell.TranslatePoint(new Point(marginX, marginY), datagrid);
        Point bottomRightPoint = bottomRightCell.TranslatePoint(
            new Point(bottomRightCell.RenderSize.Width - marginX, bottomRightCell.RenderSize.Height - marginY),
            datagrid
        );

        drawingContext.DrawRectangle(backgroundBrush, pen, new Rect(topLeftPoint, bottomRightPoint));
    }

    private static DataGridCell GetDataGridCell(DataGridCellInfo cellInfo) {
        var cellContent = cellInfo.Column.GetCellContent(cellInfo.Item);
        if (cellContent != null)
            return (DataGridCell)cellContent.Parent;
        return null;
    }
}