求救 。 看一段简单代码错误

Question

int no = 0;     
                try
                {
                    string sql = string.Format("select count(*) from Users where Id={0} and LoginPwd='{1}'", Convert.ToInt32(comboBox1.Text), textBox1.Text);
                    SqlCommand com = new SqlCommand(sql, DBHelper.con);
                    DBHelper.con.Open();
                    no = (int)com.ExecuteScalar();
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    DBHelper.con.Close();
                }
                if (no == 1)      
                {
                    int num = 0;
                    if (checkBox1.Checked)
                    {
                        try
                        {
                            string sql = string.Format("select StorageId from Storage where StorageId={0}", Convert.ToInt32(comboBox1.Text));
                            DBHelper.con.Open();
                            SqlCommand com = new SqlCommand(sql, DBHelper.con);
                            num = (int)com.ExecuteScalar();
                            
                            if (num == 1)
                            {
                                sql = string.Format("Update Storage Set LoginPwd='{0}' and State='{1}' where StorageId='{2}' ", textBox1.Text, comboBox2.Text, Convert.ToInt32(comboBox1.Text));
                                com.CommandText = sql;
                                int num2 = com.ExecuteNonQuery();
                                if (num2 > 0)
                                {
                                    MessageBox.Show("操作成功");
                                }
                            }
                            else
                            {
                                string cheng = (comboBox2.Text == "")? null:comboBox2.Text;
                                sql = string.Format("Insert into Storage(StorageId, LoginPwd, State) Values({0},'{1}','{2}')", Convert.ToInt32(comboBox1.Text), textBox1.Text,cheng);
                                com.CommandText = sql;
                                int num2 = com.ExecuteNonQuery();
                                if (num2 == 1)
                                {
                                    MessageBox.Show("插入成功");
                                }
                                }
                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }
                        finally
                        {
                            DBHelper.con.Close();
                        }
                    }

Answer 1

运行结果到这里就错误
 int num2 = com.ExecuteNonQuery();   说是   未将对象引用到设置的对象实例  别的代码没问题 。  就出现在这了 。  我找了很多遍都没找出什么原因引起的错误

Answer 2

//SqlCommand com = new SqlCommand(sql, DBHelper.con);
你应该再一次创建命令对象的实例，或者是说你应该把第一次创建的对象放在最外面（作为全局变量）//int num2 = com.ExecuteNonQuery();
在这已经访问不到com对象实例了

Answer 3

 string sql = string.Format("select StorageId from Storage where StorageId={0}", Convert.ToInt32(comboBox1.Text));
                            DBHelper.con.Open();
                            SqlCommand com = new SqlCommand(sql, DBHelper.con);
                            num = (int)com.ExecuteScalar();
                            
                            if (num == 1)
                            {
                                sql = string.Format("Update Storage Set LoginPwd='{0}' and State='{1}' where StorageId='{2}' ", textBox1.Text, comboBox2.Text, Convert.ToInt32(comboBox1.Text)); SqlCommand com = new SqlCommand(sql, DBHelper.con);                                com.CommandText = sql;
                                int num2 = com.ExecuteNonQuery();
                                if (num2 > 0)
                                {
                                    MessageBox.Show("操作成功");
                                }

Answer 4

if (num == 1)
  {
  sql = string.Format("Update Storage Set LoginPwd='{0}' and State='{1}' where StorageId='{2}' ", textBox1.Text, comboBox2.Text, Convert.ToInt32(comboBox1.Text)); SqlCommand com = new SqlCommand(sql, DBHelper.con);
  com.CommandText = sql;
  int num2 = com.ExecuteNonQuery();
  if (num2 > 0)
  {
  MessageBox.Show("操作成功");
  }