DataGridView合并单元格，方法MergeHeader重绘单元格，响应DataGridView的CellPainting事件。

        /// <summary>
        /// 合并表头,用在dataGridView的CellPainting事件中。
        /// </summary>
        /// <param name="sender">需要重绘的dataGridview</param>
        /// <param name="e">CellPainting中的参数</param>
        ///<param name="colName">列的集合(列必须是连续的，第一列放在最前面)</param>
        /// <param name="headerText">列合并后显示的文本</param>
        public void MergeHeader(object sender, DataGridViewCellPaintingEventArgs e, List<string> colNameCollection, string headerText)
        {
            if (e.RowIndex == -1)
            {
                DataGridView dg = sender as DataGridView;
                if (e.ColumnIndex < 0)
                {
                    return;
                }
                string colName = dg.Columns[e.ColumnIndex].Name;
                if (!isEnLarged)
                {
                    //0.扩展表头高度为当前的2倍
                    dg.ColumnHeadersHeightSizeMode = DataGridViewColumnHeadersHeightSizeMode.EnableResizing;
                    dg.ColumnHeadersHeight = e.CellBounds.Height * 2;
                   
                    isEnLarged = true;
                }
                if (colNameCollection.Contains(colName))
                {
                    #region 重绘列头
                    //1.计算colLen个列的区域
                    if (colNameCollection.IndexOf(colName) == 0)
                    {
                        cTop = e.CellBounds.Top;
                        cLeft = e.CellBounds.Left;

                        cWidth = e.CellBounds.Width;
                        cHeight = e.CellBounds.Height / 2;

                        foreach (string colNameItem in colNameCollection)
                        {
                            if (colNameItem.Equals(colName))
                            {//除去自己一个，加了之后colLen-1个列的宽
                                continue;
                            }
                            cWidth += dg.Columns[colNameItem].Width;
                        }
                    }

                    Rectangle cArea = new Rectangle(cLeft, cTop, cWidth, cHeight);
                    //2.把区域设置为背景色，没有列的分线及任何文字。
                    using (Brush backColorBrush = new SolidBrush(e.CellStyle.BackColor))
                    {
                        e.Graphics.FillRectangle(backColorBrush, cArea);

                    }
                    //3.绘制新列头的边框
                    using (Pen gridPen = new Pen(dg.GridColor))
                    {
                        //3.1 上部边框
                        e.Graphics.DrawLine(gridPen, cLeft, cTop, cLeft + cWidth, cTop);
                        using (Pen hilightPen = new Pen(Color.WhiteSmoke))
                        {
                            //3.2 顶部高光
                            e.Graphics.DrawLine(hilightPen, cLeft, cTop + 1, cLeft + cWidth, cTop + 1);
                            //3.3 左部反光线
                            e.Graphics.DrawLine(hilightPen, cLeft, cTop + 3, cLeft, cTop + cHeight - 2);
                        }
                        //3.4 下部边框
                        e.Graphics.DrawLine(gridPen, cLeft, cTop + cHeight - 1, cLeft + cWidth, cTop + cHeight - 1);
                        //3.5 右部边框                        
                        e.Graphics.DrawLine(gridPen, cLeft + cWidth - 1, cTop, cLeft + cWidth - 1, cTop + cHeight);//(cTop+cHeight)/2);
                    }

                    //4.写文本
                    //不是第一列则不写文字。
                    int wHeadStr = (int)(headerText.Length * e.CellStyle.Font.SizeInPoints);
                    int wHeadCell = (cWidth - wHeadStr) / 2;
                    int pHeadLeft = wHeadCell + cLeft;
                  
                    using (Brush foreBrush = new SolidBrush(e.CellStyle.ForeColor))
                    {
                        e.Graphics.DrawString(headerText, e.CellStyle.Font, foreBrush, new PointF(pHeadLeft, cTop + 3));

                    }

                    //5 绘制子列背景
                    int FatherColHeight = e.CellBounds.Height / 2;//上面一行的高度
                    using (Brush backColorBrush = new SolidBrush(e.CellStyle.BackColor))
                    {
                        e.Graphics.FillRectangle(backColorBrush, new Rectangle(e.CellBounds.X, e.CellBounds.Y + FatherColHeight, e.CellBounds.Width - 1, e.CellBounds.Height / 2 - 1));
                    }
                    //5.1绘制子列的边框                   
                    using (Pen gridPen = new Pen(dg.GridColor))
                    {
                        using (Pen hilightPen = new Pen(Color.WhiteSmoke))
                        {
                            //5.2 左部反光线
                            e.Graphics.DrawLine(hilightPen, cLeft, cTop + 3 + FatherColHeight, cLeft, cTop + cHeight - 2 + FatherColHeight);
                        }
                        //5.3 下部边框
                        e.Graphics.DrawLine(gridPen, cLeft, cTop + cHeight - 1 + FatherColHeight, cLeft + cWidth, cTop + cHeight - 1 + FatherColHeight);

                        //5.4 右部边框
                        e.Graphics.DrawLine(gridPen, e.CellBounds.X + e.CellBounds.Width - 1, e.CellBounds.Top + FatherColHeight, e.CellBounds.X + e.CellBounds.Width - 1, e.CellBounds.Top + e.CellBounds.Height + FatherColHeight);//(cTop+cHeight)/2);                    

                    }
                    //5.5 写子列的文本
                    int wStr = (int)(dg.Columns[e.ColumnIndex].HeaderText.Length * e.CellStyle.Font.SizeInPoints);
                    int wCell = e.CellBounds.Width;
                    int pLeft = (wCell - wStr) / 2;//相对CELL左边框的左坐标

                    using (Brush foreBrush = new SolidBrush(e.CellStyle.ForeColor))
                    {
                        e.Graphics.DrawString(dg.Columns[e.ColumnIndex].HeaderText, e.CellStyle.Font, foreBrush, new PointF(e.CellBounds.X + pLeft, cTop+5 + FatherColHeight));
                    }
                    #endregion            
                    e.Handled = true;
                }
            }
        }