WinForms Data Grid - Select multiple rows by dragging the mouse in RowSelect mode

In GridMultiSelectMode.CellSelect mode, users can select a contiguous block of cells by dragging the mouse over them. To select multiple rows in the same way in GridMultiSelectMode.RowSelect mode, create a GridView descendant and override GridHandler.OnMouseDown and GridHandler.OnMouseMove methods:

protected override bool OnMouseDown(MouseEventArgs ev) {
    var result = base.OnMouseDown(ev);
    if (ev.Button == MouseButtons.Left && Control.ModifierKeys == Keys.None && View.IsRowSelect && DownPointHitInfo.InRowCell)
        mouseDownHitInfo = DownPointHitInfo;
    else
        mouseDownHitInfo = null;
    return result;
}
protected override bool OnMouseMove(MouseEventArgs ev) {
    var result = base.OnMouseMove(ev);
    if (!result && ev.Button == MouseButtons.Left && mouseDownHitInfo != null) {
        var dragSize = SystemInformation.DragSize;
        var dragRect = new Rectangle(
            new Point(mouseDownHitInfo.HitPoint.X - dragSize.Width / 2, mouseDownHitInfo.HitPoint.Y - dragSize.Height / 2),
            dragSize);
        if (!dragRect.Contains(ev.Location)) {
            mouseDownHitInfo = null;
            View.StartAccessSelection();
            result = true;
        }
    }
    return result;
}

Files to look at:

• CustomGrid.cs (VB: CustomGrid.vb)
• Form1.cs (VB: Form1.vb)

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