private void dataGridView1_CellMouseDown(object sender, DataGridViewCellMouseEventArgs e)
        {
            if (e.RowIndex == -1 && e.ColumnIndex == -1)//单击表头
            {
                bool checkAll = false;
                for (int i = 0; i < dataGridView1.Rows.Count; i++)
                {
                    if (dataGridView1.Rows[i].Cells[0].Value == null)//没有给checkBox列激活即选中的情况下，直接获取值.ToString会报错
                    {
                        checkAll = false;//checkBox列还未激活，即都还为选中
                        break;
                    }
                    else if (dataGridView1.Rows[i].Cells[0].Value.ToString() == "True")
                    {
                        string a = dataGridView1.Rows[i].Cells[0].Value.ToString();
                        checkAll = true;
                    }
                    else if (dataGridView1.Rows[i].Cells[0].Value.ToString() != "True")
                    {
                        checkAll = false;//有未被选中的记录，则标记全选为false，跳出循环
                        break;
                    }

                }
                if (checkAll == false) //存在未选的记录，则更新为全选
                {
                    for (int i = 0; i < dataGridView1.Rows.Count; i++)
                    {
                        dataGridView1.Rows[i].Cells[0].Value = true;
                    }
                }
                else //如果原为全选状态，则更新为全不选
                {
                    for (int i = 0; i < dataGridView1.Rows.Count; i++)
                    {
                        dataGridView1.Rows[i].Cells[0].Value = false;
                    }
                    //下面的代码都无法彻底清空行选中状态（不是勾上）
                    dataGridView1.ClearSelection();
                    dataGridView1.CurrentCell = null;
                 
                    // dataGridView1.Rows[0].Selected = false;
                }
            }
        }