Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

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Hello people hope you are well. I am designing a login website in C# and Asp.net, my code is building succesfully with no errors, and when i run it it takes me to the login page, bt when i try to login with my credentials that are in e data base it s giving me the following error.

Server Error in '/Loginwebsite' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

C#

Line 17:     {
Line 18:         
Line 19:         SqlConnection con = new SqlConnection (ConfigurationManager.ConnectionStrings["ReConnectionString"].ConnectionString);
Line 20:         con.Open(); 
Line 21:         string cmdStr = "Select count(*) from Registration where username'" + TextBox1.Text + "'";

Source File: c:\Users\Jinda Guru\Documents\Visual Studio 2010\WebSites\Loginwebsite\Login.aspx.cs Line: 19

please help me out guys...thank you

Posted 12-Sep-13 4:24am

simsmats

Updated 12-Sep-13 4:25am

v2

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Abey Thomas 12-Sep-13 10:32am

Have you defined the Connection String in your web.config?
Another problem I see in your code is the sql query. You are missing an = (equals to) symbol

string cmdStr = "Select count(*) from Registration where username='" + TextBox1.Text + "'";

Richard C Bishop 12-Sep-13 10:52am

Your query leaves you susceptible to SQL injection attacks. Always use parameterized queries.

4 solutions

Solution 4

ok thank you let me try these two things and see if it fixes it

Posted 16-Sep-13 0:33am

simsmats

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-09-12T05:29:00

Most likely, ConfigurationManager.ConnectionStrings["ReConnectionString"] returns null. It's bad that the immediate constant string in hard-coded in this sample is hard-coded; this is a permanent source of bugs; if you misspell, the compiler won't warn you, so you should never unconditionally dereference objects obtained through such properties.

And you should not ask a question every time you face such a simple situation. Instead, you should develop the elementary skills of dealing with them by yourself.

This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced 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

Dave Kreskowiak · Accepted Answer · 2013-09-12T04:29:00

Solution 1

It's easy. ConfigurationManager.ConnectionStrings["..."] returned null. You possibly don't have a connection string named "ReConnectionString" in web.config??

Posted 12-Sep-13 4:29am

Dave Kreskowiak

Abey Thomas · Accepted Answer · 2013-09-12T04:32:00

Solution 2