System.NullReferenceException: '未将对象引用设置为对象的实例。' 这里的问题是什么？

Question

错误说 System.Data.Common.DbCommand.ExecuteScalar(...) 返回 null。

protected void Button3_OnClick(object sender, EventArgs e)
            {
    
                SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegisterConnectionString"].ConnectionString);
            conn.Open();
            string checkuser = "select count(*) from [User] where emailAddress='" + TextBox6.Text + "'";
            SqlCommand cmd = new SqlCommand(checkuser, conn);
            int temp = Convert.ToInt32(cmd.ExecuteScalar().ToString());
            conn.Close();
            if (temp >= 1)
            {
                conn.Open();
                string checkPasswordQuery = "select password from [User] where emailAddress='" + TextBox7.Text + "'";
                SqlCommand Passcmd = new SqlCommand(checkPasswordQuery, conn);
                string password = Passcmd.ExecuteScalar().ToString();
                if (password == TextBox7.Text)
                {
                    Session["New"] = TextBox6.Text;
                    Response.Write("Password is correct");
                    Response.Redirect("WebForm1.aspx");
                }
                else
                    Response.Write("Password is incorrect");

            }

            else
                Response.Write("Username not found");
        }
    }
}

据说错误发生在写成的行上： string password = Passcmd.ExecuteScalar().ToString();
以下是我的 Web 配置：

 <configuration> <connectionStrings> <add name="RegisterConnectionString" connectionString="Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\\User.mdf;Integrated Security=True" providerName="System.Data.SqlClient" /> </connectionStrings> <system.web> <compilation debug="true" targetFramework="4.7.2" /> <httpRuntime targetFramework="4.7.2" /> </system.web> <system.webServer>

Answer 1

您收到错误是因为您在 ExecuteScaler 上调用 .ToString() 已返回“null”。

基于此，问题是为什么它给你“空”？

根据我们的代码，以下内容有些推测性，但是......

您的第一个查询（似乎有效）使用“TextBox6.Text”作为电子邮件地址，而您的第二个查询（不起作用）使用“TextBox7.Text”作为电子邮件地址。

如果这不是故意的（即如果 TextBox6.Text 是电子邮件地址的正确框），那么我建议您的第二个查询应该是：

string checkPasswordQuery = "select password from [User] where emailAddress='" + TextBox6.Text + "'";

Answer 2

尝试注释掉代码中的第二个 conn.Open() 和 con.close()。

protected void Button3_OnClick(object sender, EventArgs e)
            {
    
                SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegisterConnectionString"].ConnectionString);
            conn.Open();
            string checkuser = "select count(*) from [User] where emailAddress='" + TextBox6.Text + "'";
            SqlCommand cmd = new SqlCommand(checkuser, conn);
            int temp = Convert.ToInt32(cmd.ExecuteScalar().ToString());
            // conn.Close();  ********* comment out ***********
            if (temp >= 1)
            {
                // conn.Open();  ********* comment out ***********
                string checkPasswordQuery = "select password from [User] where emailAddress='" + TextBox7.Text + "'";
                SqlCommand Passcmd = new SqlCommand(checkPasswordQuery, conn);
                string password = Passcmd.ExecuteScalar().ToString();
                if (password == TextBox7.Text)
                {
                    Session["New"] = TextBox6.Text;
                    Response.Write("Password is correct");
                    Response.Redirect("WebForm1.aspx");
                }
                else
                    Response.Write("Password is incorrect");

            }

            else
                Response.Write("Username not found");
        }
    }
}