c# - 你调用的对象是空的。执行标量()

Question

我总是收到“对象引用未设置到对象的实例”。我不知道我做错了什么。我确实尝试执行查询:“select count(*) from Registration where Username='Myname' and it returned 1;

代码:

 protected void Button1_Click(object sender, EventArgs e)
    {

            DBConnect con = new DBConnect();
            string query = "select count(*) from Registration where Username='" + TextBox1.Text + "'";

            //Open connection
            if (con.OpenConnection() == true)
            {
                //Create Command
                MySqlCommand cmd = new MySqlCommand(query, con.Initialize());
                //Create a data reader and Execute the command

                int temp = Convert.ToInt32(cmd.ExecuteScalar().ToString());

                if (temp == 1)
                {
                    string query1 = "Select Password from Registration where Username='" + TextBox1.Text + "'";
                    MySqlCommand pass = new MySqlCommand(query1, con.Initialize());
                    string password = pass.ExecuteScalar().ToString();

                    if (password == TextBox2.Text)
                    {
                        Session["new"] = TextBox1.Text;
                        Response.Redirect("GreenhouseHomepage.aspx");
                    }
                    else
                    {
                        Label1.Text = "Invalid Password...!!";
                        Label1.Visible = true;
                    }
                }
                else
                {
                    Label1.Text = "Invalid Username...!!";
                    Label1.Visible = true;
                }
            }

            //close Connection
            con.CloseConnection();           
    }

错误:

Answer 1

试一试。

//Open connection
            if (con.OpenConnection() == true)
            {
                // Since you know that the connection is open, you can just 
                // pass the entire conn object
                MySqlCommand cmd = new MySqlCommand(query, con);
                //Create a data reader and Execute the command

                int temp = Convert.ToInt32(cmd.ExecuteScalar());

如果未找到任何记录，COUNT(*) 将返回 0，因此您始终会从语法中获取一个值。