DataGridView 合并相邻单元格（同一列中值相同）

System.NullReferenceException:“未将对象引用设置到对象的实例。” 
System.Windows.Forms.DataGridViewCellPaintingEventArgs.Value.get 返回 null。

合并单元格可以参考
https://www.cnblogs.com/bloodofhero/archive/2012/07/05/2578068.html

合并单元格函数

        /// 

        /// 合并同一列中值相同的相邻单元格
        /// 
        /// DataGridView
        /// 要合并的列的索引列表
        /// 当前单元格的属性访问器
        private void MergeCellInOneColumn(DataGridView dgv, List columnIndexList, DataGridViewCellPaintingEventArgs e)
        {
            if (columnIndexList.Contains(e.ColumnIndex) && e.RowIndex != -1)
            {
                Brush gridBrush = new SolidBrush(dgv.GridColor);
                Brush backBrush = new SolidBrush(e.CellStyle.BackColor);
                Pen gridLinePen = new Pen(gridBrush);

                //擦除
                e.Graphics.FillRectangle(backBrush, e.CellBounds);

                //画右边线
                e.Graphics.DrawLine(gridLinePen,
                   e.CellBounds.Right - 1,
                   e.CellBounds.Top,
                   e.CellBounds.Right - 1,
                   e.CellBounds.Bottom - 1);

                //画底边线
                if ((e.RowIndex < dgv.Rows.Count - 1 && dgv.Rows[e.RowIndex + 1].Cells[e.ColumnIndex].Value.ToString() != e.Value.ToString()) ||
                    e.RowIndex == dgv.Rows.Count - 1)
                {
                    e.Graphics.DrawLine(gridLinePen,
                        e.CellBounds.Left,
                        e.CellBounds.Bottom - 1,
                        e.CellBounds.Right - 1,
                        e.CellBounds.Bottom - 1);
                }

                //写文本
                if (e.RowIndex == 0 || dgv.Rows[e.RowIndex - 1].Cells[e.ColumnIndex].Value.ToString() != e.Value.ToString())
                {
                    e.Graphics.DrawString((String)e.Value, e.CellStyle.Font,
                        Brushes.Black, e.CellBounds.X + 2,
                        e.CellBounds.Y + 5, StringFormat.GenericDefault);
                }

                e.Handled = true;
            }
        }

引用方法

 private void dataGridView1_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
        {
            List indexs = new List() { 0, 1 };
            MergeCellInOneColumn(dataGridView1, indexs, e);
        }