Dragging items in a ListBox control with visual insertion guides

private void DrawInsertionLine()
{
  if (this.InsertionIndex != InvalidIndex)
  {
    int index;

    index = this.InsertionIndex;

    if (index >= 0 && index < this.Items.Count)
    {
      Rectangle bounds;
      int x;
      int y;
      int width;

      bounds = this.GetItemRectangle(this.InsertionIndex);
      x = 0; // aways fit the line to the client area, regardless of how the user is scrolling
      y = this.InsertionMode == InsertionMode.Before ? bounds.Top : bounds.Bottom;
      width = Math.Min(bounds.Width - bounds.Left, this.ClientSize.Width); // again, make sure the full width fits in the client area

      this.DrawInsertionLine(x, y, width);
    }
  }
}

protected void Invalidate(int index)
{
  if (index != InvalidIndex)
  {
    Rectangle bounds;

    bounds = this.GetItemRectangle(index);
    if (this.InsertionMode == InsertionMode.Before && index > 0)
    {
      bounds = Rectangle.Union(bounds, this.GetItemRectangle(index - 1));
    }
    else if (this.InsertionMode == InsertionMode.After && index < this.Items.Count - 1)
    {
      bounds = Rectangle.Union(bounds, this.GetItemRectangle(index + 1));
    }

    this.Invalidate(bounds);
  }
}

protected int DragIndex { get; set; }

protected Point DragOrigin { get; set; }

protected override void OnMouseDown(MouseEventArgs e)
{
  base.OnMouseDown(e);

  if (e.Button == MouseButtons.Left)
  {
    this.DragOrigin = e.Location;
    this.DragIndex = this.IndexFromPoint(e.Location);
  }
  else
  {
    this.DragOrigin = Point.Empty;
    this.DragIndex = InvalidIndex;
  }
}```

When the user first presses a button, I record both the position of the cursor and which item is under it.

```csharp
protected override void OnMouseMove(MouseEventArgs e)
{
  if (this.AllowItemDrag && !this.IsDragging && e.Button == MouseButtons.Left && this.IsOutsideDragZone(e.Location))
  {
    this.IsDragging = true;
    this.DoDragDrop(this.DragIndex, DragDropEffects.Move);
  }

  base.OnMouseMove(e);
}

private bool IsOutsideDragZone(Point location)
{
  Rectangle dragZone;
  int dragWidth;
  int dragHeight;

  dragWidth = SystemInformation.DragSize.Width;
  dragHeight = SystemInformation.DragSize.Height;
  dragZone = new Rectangle(this.DragOrigin.X - (dragWidth / 2), this.DragOrigin.Y - (dragHeight / 2), dragWidth, dragHeight);

  return !dragZone.Contains(location);
}

protected override void OnDragOver(DragEventArgs drgevent)
{
  if (this.IsDragging)
  {
    int insertionIndex;
    InsertionMode insertionMode;
    Point clientPoint;

    clientPoint = this.PointToClient(new Point(drgevent.X, drgevent.Y));
    insertionIndex = this.IndexFromPoint(clientPoint);

    if (insertionIndex != InvalidIndex)
    {
      Rectangle bounds;

      bounds = this.GetItemRectangle(insertionIndex);
      insertionMode = clientPoint.Y < bounds.Top + (bounds.Height / 2) ? InsertionMode.Before : InsertionMode.After;

      drgevent.Effect = DragDropEffects.Move;
    }
    else
    {
      insertionIndex = InvalidIndex;
      insertionMode = InsertionMode.None;

      drgevent.Effect = DragDropEffects.None;
    }

    if (insertionIndex != this.InsertionIndex || insertionMode != this.InsertionMode)
    {
      this.Invalidate(this.InsertionIndex); // clear the previous item
      this.InsertionMode = insertionMode;
      this.InsertionIndex = insertionIndex;
      this.Invalidate(this.InsertionIndex); // draw the new item
    }
  }

  base.OnDragOver(drgevent);
}

protected override void OnDragLeave(EventArgs e)
{
  this.Invalidate(this.InsertionIndex);
  this.InsertionIndex = InvalidIndex;
  this.InsertionMode = InsertionMode.None;

  base.OnDragLeave(e);
}

protected override void OnDragDrop(DragEventArgs drgevent)
{
  if (this.IsDragging)
  {
    try
    {
      if (this.InsertionIndex != InvalidIndex)
      {
        int dragIndex;
        int dropIndex;

        dragIndex = (int)drgevent.Data.GetData(typeof(int));
        dropIndex = this.InsertionIndex;

        if (dragIndex < dropIndex)
        {
          dropIndex--;
        }
        if (this.InsertionMode == InsertionMode.After && dragIndex < this.Items.Count - 1)
        {
          dropIndex++;
        }

        if (dropIndex != dragIndex)
        {
          object dragItem;

          dragItem = this.Items[dragIndex];

          this.Items.Remove(dragItem);
          this.Items.Insert(dropIndex, dragItem);
          this.SelectedItem = dragItem;
        }
      }
    }
    finally
    {
      this.Invalidate(this.InsertionIndex);
      this.InsertionIndex = InvalidIndex;
      this.InsertionMode = InsertionMode.None;
      this.IsDragging = false;
    }
  }

  base.OnDragDrop(drgevent);
}