How Do I Prevent This Error On Timer

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The application is running absolutely fine however when i lock my PC and the timer ticks (the tick is set at 60 seconds) then this error is seen. After this the application does not work even if i click continue.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
at ScreenCatcher.Form1.timer_capture_Tick(Object sender, EventArgs e)
at System.Windows.Forms.Timer.OnTick(EventArgs e)
at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

code is as follows:

C#

private void timer_capture_Tick(object sender, EventArgs e)
        {
            try
            {
                timer_capture.Enabled = false;

                EmployeeDirectoryExists();

                DateDirectoryExists();

                if (EmpDirExists == true) //employee folder exist true
                {
                    if (DateDirExists == true)//date folder exist true
                    {
                        capture();
                        UploadFile();
                    }

                    else //date folder exist false
                    {
                        createDateDirectory();
                        capture();
                        UploadFile();

                    }

                }
                else //employee folder exist false
                {
                    createEmployeeDirectory();
                    createDateDirectory();
                    capture();
                    UploadFile();
                }

                timer_capture.Enabled = true;
            }

            catch (Exception ex)
            {
                string LogFileName = String.Format("{0:dd-MM-yyyy}__{1}", DateTime.Now, "uploadFile.txt");
                StreamWriter sr = new StreamWriter(LocalTempFolder+ "/" + LogFileName, true);
                sr.WriteLine(" ---*****---");
                sr.WriteLine("Error uploading the file  " + FileName);
                sr.WriteLine("error is " + ex.InnerException.ToString());
                sr.Close();
            }
        }

Posted 16-Nov-14 17:53pm

Hassan(Aych Jay)

Updated 16-Nov-14 18:11pm

v2

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Comments

PIEBALDconsult 17-Nov-14 0:01am

You would have to show us the code involved. I suspect a cross-thread issue.

Hassan(Aych Jay) 17-Nov-14 0:05am

let me edit the question and add the code in it

Philippe Mori 17-Nov-14 0:28am

You have to tell us at which line... And by the way a debugger will tell you that and once that information is known, you should be able to find the problem by yourself by inspecting variables with a debugger.

Hassan(Aych Jay) 17-Nov-14 0:40am

As i mentioned earlier the code is running perfectly. when i lock my PC for a minute then the tick fails. then and only the when the PC is locked. So how is it possible to debug this scenario ? Please help me understand so that i can try that

Philippe Mori 17-Nov-14 9:03am

If you run the application under the debugger and tell it to stop on all exception and then lock the PC, then it should throw that exception as usual.

In particular, if InnerException is null then the above code would give the mentionned error and you would not be able to use the log again.

Bernhard Hiller 17-Nov-14 2:19am

sr.WriteLine("error is " + ex.InnerException.ToString());
Why InnerException? What's the exception proper?

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2014-11-16T20:00:00

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

How Do I Prevent This Error On Timer

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