如何将excel导入到datagrid，然后将特定的列保存到mysql数据库

我想添加我导入到数据gridview的excel文件的特定列。 但问题是，当我点击保存我得到以下错误：

“无法将types为”System.DBNull“的对象键入”System.String“。

我总共有9列，我想只添加3列。 这是我的代码：

private void button1_Click(object sender, EventArgs e) { OpenFileDialog dlg = new OpenFileDialog(); dlg.Filter = "Excel Files|*.xls;*.xlsx;*.xlsm"; if (dlg.ShowDialog() == DialogResult.OK) { label1.Text = dlg.FileName.ToString(); string path = label1.Text; string conn = @"Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=" + path + ";" + "Extended Properties=" + "\"" + "Excel 12.0;HDR=YES;" + "\""; OleDbConnection conn1 = new OleDbConnection(conn); OleDbDataAdapter dataAdapter = new OleDbDataAdapter("select * from [Sheet1$]", conn1); DataTable dataSet = new DataTable(); dataAdapter.Fill(dataSet); dataGridView1.DataSource = dataSet; } } private void button2_Click(object sender, EventArgs e) { using (MySqlConnection conn = new MySqlConnection(cs)) { MySqlCommand command = new MySqlCommand(); MySqlDataReader reader; MySqlCommand command2 = new MySqlCommand(); //MySqlDataReader reader1; using (MySqlCommand cmd = new MySqlCommand()) { cmd.Connection = conn; conn.Open(); for (int i = 0; i < dataGridView1.Rows.Count; i++) { Boolean Empty = true; for (int j = 0; j < dataGridView1.Columns.Count; j++) { if ((string)dataGridView1.Rows[i].Cells[j].Value != "") { Empty = false; StrQuery = @"INSERT Ignore INTO course(Code,Course,Major) VALUES ('" + dataGridView1.Rows[i].Cells["Course Code"].Value + "','" + dataGridView1.Rows[i].Cells["Course"].Value + "','" + dataGridView1.Rows[i].Cells["Major"].Value + "')"; cmd.CommandText = StrQuery; cmd.ExecuteNonQuery(); } } if (Empty) { dataGridView1.Rows.RemoveAt(i); i--; } } } MessageBox.Show("Successfully Added!"); } }