How Do I Solve This?

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Object reference not set to an instance of an object.

VB

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
       /*error is here/
 Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("log").ConnectionString)
        con.Open()
        Dim cmd As New SqlCommand("select * from log where UserName=@username and Password=@password", con)
        cmd.Parameters.AddWithValue("@username", TextBox1.Text)
        cmd.Parameters.AddWithValue("@password", TextBox2.Text)
        Dim da As New SqlDataAdapter(cmd)
        Dim dt As New DataTable()
        da.Fill(dt)
        If dt.Rows.Count > 0 Then
            Response.Redirect("list.aspx")
        Else
            ClientScript.RegisterStartupScript(Page.[GetType](), "validation", "<script language='javascript'>alert('Invalid Username and Password')</script>")
        End If
    End Sub

Posted 31-Jan-15 5:47am

Member 11417068

Updated 31-Jan-15 6:01am

User 11060979

v4

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Sergey Alexandrovich Kryukov 31-Jan-15 13:34pm

In what line? You comment is between line, but I guess it is about the line below, right?
—SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-01-31T07:39:00

Most likely, ConnectionStrings("log") returns null and then you try to dereference this null object by taking its property value ConnectionString, which throws this exception. It's really important to learn how to debug such simple problems all by yourself. You cannot ask questions every time it happens.

You did not precisely pin-point the source of exception. 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:
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