I cant see where I am going wrong, can any one help please

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Object reference not set to an instance of an object.

I keep getting the above error but cant seem to see the issue. Here is my code if anyone can help please.

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;


public partial class Login : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnLogin_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["CarValetConnectionString"].ConnectionString);
        con.Open();
        string checkUser = "select count(*) from Registered_Customers where Username='" + txtUsername.Text + "'";
        SqlCommand com = new SqlCommand(checkUser, con);
        int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
        con.Close();

        //varifying nusername and password
        if (temp == 1)
        {
            con.Open();
            string checkPasswordQuery = "select Password from NewUserRegistration where Username='" + txtUsername.Text + "'";
            SqlCommand passCom = new SqlCommand(checkUser, con);
            string password = passCom.ExecuteScalar().ToString().Replace(" ","");
            // Varifing password
            if (password == txtPassword.Text)
            {
                Session["New"] = txtUsername.Text;
                Response.Write("Password is correct");
                Response.Redirect("DMembersOnly.aspx");
            }
            else
            {
                Response.Write("Password is not correct");
            }
        }
        else
        {
            Response.Write("Username is not correct");
        }
        

    }

}

What I have tried:

Checking my connection in my sql server

Posted 5-Apr-16 3:57am

neville1967

Updated 5-Apr-16 6:02am

F-ES Sitecore

v2

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F-ES Sitecore 5-Apr-16 10:03am

This question is asked multiple times a day, did you google the error message you're seeing?

1 When you get an error always say what line the error is on
2 Use the debugger to see what the state of your variables etc is and see what is "null" that you're trying to reference.
3 Use parameterised queries to execute your SQL and not string concatenation

If I had to guess I'd say the issue is that ConfigurationManager.ConnectionStrings["CarValetConnectionString"] is null because you have no such connection string defined in the correct place of your web.config file, so that's the first thing to check.

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OriginalGriff · Answer 1 · 2016-04-05T04:03:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Sergey Alexandrovich Kryukov · Answer 2 · 2016-04-05T04:12:00

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferences by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object".

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx,
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx.

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx.

Good luck,

—SA

CHill60 · Answer 3 · 2016-04-05T06:02:00

In your Solution Explorer locate "App.Config".

If it doesn't exist then right-click on your project name, Add, New Item and choose "Application Configuration File" from the list of options. This will create the App.Config file for your project.

In that file you need a <connectionstrings></connectionstrings>section that contains details of your CarValetConnectionString.

E.g.

XML

<configuration>
  <connectionstrings>
    <add name="CarValetConnectionString">
        connectionString="Data Source=YourServer;Initial Catalog=YourDataBase;Integrated Security=True"
        providerName="System.Data.SqlClient" />
  </add></connectionstrings>
</configuration>

I cant see where I am going wrong, can any one help please

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