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I am trying to allowing users to change connectionstring at runtime vb .net 2010
The code for this is shown below but it raises runtime error "Object reference not set to an instance of an object.
'Constructing new connectionstring from the inputs
            Dim Con As New StringBuilder("Data Source=")
            Con.Append(";Initial Catalog=")
            Con.Append(";Integrated Security=SSPI;")
            Dim strConn As String = Con.ToString()

Public Sub ChangeConnectionString(strCon As String)
            Dim config As Configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None)
           config.ConnectionStrings.ConnectionStrings("cdataConnectionString1").ConnectionString = strCon
            'Save to file
            'force changes to take effect so that we can start using
            'this new connection string immediately
    End Sub

I cannot figure out the problem is.

Any assistance would be greatly appreciated

Thanks in advance
Richard C Bishop 28-May-13 17:34pm    
That exception means you are attempting to use an object that has not been created. You need to debug your application and figure out where that is coming from.

1 solution

First of all, your approach is weird, to say the list. Configuration files are designed to be read once at application startup. Why would you ever write into it? Why do you think it will be rewritten? Even if you make it rewrite, it would be pointless, because you can simply pass the modified content where you use it. In particular, did you look at connection string builders? Please see:[^].

Now, about the exception you face…

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 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:[^],[^].

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:[^].

Good luck,
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noblepaulaziz 28-May-13 19:07pm    
Thanks so much for your assistance, Sergey
Sergey Alexandrovich Kryukov 28-May-13 19:28pm    
Sure. Will you accept the answer formally (green button)?

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