Object reference not set to an instance of an object - Error

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Private Sub SetPrinterName(ByVal PrinterName As String, ByVal AppPath As String)
		Dim prnPrinter As Printer

		For Each prnPrinter In Printers
			If prnPrinter.DeviceName = PrinterName Then
                p = prnPrinter
				Exit For
			End If
		Next
        p.DocumentName = "ERP System" ---Some here i get this error "Object reference not set to an instance of an object."

What im i doing wrong please?

		Me.Path = AppPath
		If bIsDebug Then
            p.PrintAction = Printing.PrintAction.PrintToPreview
		End If
	End Sub
#End Region
#Region "Images"
	Public Property Path() As String
		Get
			Return _Path
		End Get
		Set(ByVal value As String)
			_Path = value
		End Set
	End Property

Posted 11-Sep-13 4:26am

Benedict Ibenezim

Updated 11-Sep-13 4:28am

Richard C Bishop

v3

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Comments

[no name] 11-Sep-13 10:29am

"What im i doing wrong please", you are assuming that "p" is a valid object reference.

Richard C Bishop 11-Sep-13 10:29am

Where is "p" coming from?

TryAndSucceed 11-Sep-13 10:32am

Debug and see which line throws an exception. And let us know.

2 solutions

Solution 2

In simple words: the printer was not found. Hence p was never set to a valid printer, and is still Nothing.
Before the line which throws the error, add three extra lines similar to (I am not so sure with VB syntax...):

VB

If p Is Nothing Then
    throw new ApplicationException(string.Format("The printer {0} could not be found", PrinterName))
End If

and you'll get an error message which is easy to understand for you and the user.

Posted 11-Sep-13 21:19pm

Bernhard Hiller

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-09-11T04:34:00

Please see the comments to the question.

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

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