My Visual Basic project gives me the error "Null reference exception was unhandeled" and also says "Object reference not set to a reference of an object" Any ideas?

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VB

Private Sub btnRatio_Click(sender As Object, e As EventArgs) Handles btnRatio.Click

    ReDim Crater(NumCrat).Ratio(NumSch)
    ReDim Ratios(NumCrat, NumSch)

    Dim count As Integer
    For count = 1 To NumCrat
        Dim count2 As Integer
        For count2 = 1 To NumSch

            grdAll.Col = count
            grdAll.Row = count2
            Ratios(count, count2) = CDbl(InputBox("Please enter the student/textbook -> ratio of school " + CStr(count2) + " crater " + CStr(count)))
            grdAll.Text = CStr(Ratios(count, count2))

            Crater(count).Ratio(count2) = Ratios(count, count2)
        Next
    Next
End Sub

Posted 10-Apr-14 12:50pm

Member 10740537

Updated 10-Apr-14 23:09pm

Richard MacCutchan

v2

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[no name] 10-Apr-14 18:54pm

No ideas needed. You are trying to use an object that is null. Or trying to access the methods or properties of an object that is null. The error message tells you that.

Sergey Alexandrovich Kryukov 10-Apr-14 19:52pm

In what line?
—SA

Richard MacCutchan 11-Apr-14 5:12am

There are so many variables in the above code that are outside this subroutine it's impossible to guess which one does not have a valid reference.

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Sergey Alexandrovich Kryukov · Answer 1 · 2014-04-10T13:52:00

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 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