string strConn = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=F:\板材切割\Part.mdb";
            OleDbConnection conn = new OleDbConnection(strConn);
            conn.Open();
            List<string> sqls = new List<string>();
            try
            {
                for (int i = 0; i < dataGridView1.RowCount; i++)
                {
                    Part_ID = dataGridView1.Rows[i].Cells[0].Value.ToString();
                    Length = Convert.ToSingle(dataGridView1.Rows[i].Cells[1].Value.ToString());
                    Width = Convert.ToSingle(dataGridView1.Rows[i].Cells[2].Value.ToString());
                    Depth = Convert.ToSingle(dataGridView1.Rows[i].Cells[3].Value.ToString());
                    Num = Convert.ToInt16(dataGridView1.Rows[i].Cells[4].Value.ToString());
                    Depict = dataGridView1.Rows[i].Cells[5].Value.ToString();



                    //string sql = string.Format("insert into Part values('{0}')", dataGridView1[0, i].Value .ToString());
                    string sql = "insert into Part(Part_ID,Length,Width,Depth,Num,Depict) values('" + Part_ID + "' ," + Length + " ," + Width + " ," + Depth + " ," + Num + " ,'" + Depict + "');";

                    //string sql = "insert into Part(Part_ID,Length,Width,Depth,Num,Depict) values('" + dataGridView1.Rows[i].Cells[0].Value.ToString() + "' ," + Convert.ToSingle(dataGridView1.Rows[i].Cells[1].Value.ToString()) + " ," + Convert.ToSingle(dataGridView1.Rows[i].Cells[2].Value.ToString()) + " ," + Convert.ToSingle(dataGridView1.Rows[i].Cells[3].Value.ToString()) + " ," + Convert.ToInt16(dataGridView1.Rows[i].Cells[4].Value.ToString()) + " ,'" + dataGridView1.Rows[i].Cells[5].Value.ToString() + "');";
                    sqls.Add(sql);
                }
            }
            catch (NullReferenceException ne)
            { MessageBox.Show(ne.Message.ToString()); }
            
            //connection=new SqlConnection(...);
           OleDbCommand  command = new OleDbCommand();
            command.Connection = conn;
            //connection.Open();
            OleDbTransaction   transaction = conn.BeginTransaction();
            command.Transaction = transaction;//创建事务
            try
            {
                foreach (string sql in sqls)
                {
                    command.CommandText = sql;
                    command.ExecuteNonQuery();
                }
                 //command.Transaction.Commit();
                transaction.Commit();//提交事务
            }
            catch (Exception)
            {
                transaction.Rollback();//事务回滚，如果出错，恢复到操作前的状态
                MessageBox.Show("未能保存");
            }
            finally
            {
                conn.Close();
            }
        }
我这个方法不对吗，为什么总是提示“未将对象引用设置到对象实例”