object reference not set to an instance of an object VB.NET

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i need some help object reference not set to an instance of an object error in my code
here is my code

VB

Public Function Add(std As DataTable) As IList
      Try
          listr.Add(std.Rows)
          Return listr
      Catch ex As Exception
          MsgBox("Can't load Web page" & vbCrLf & ex.Message)
      End Try

  End Function

Posted 24-Jan-16 8:36am

Member 12284416

Updated 24-Jan-16 8:54am

Patrice T

v2

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Patrice T 24-Jan-16 14:57pm

Your code is not complete.
There should be some other code about variable listr

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Sergey Alexandrovich Kryukov · Answer 1 · 2016-01-24T10:02:00

The whole idea is bad. Of course, you have to initialize listr. But then, the idea of returning it from the function is wrong. If your function is used in the same instance of the same class, you already have this instance, and if you call this function from outside, what are you adding to? You have to add something to a collection instance which is already known to the caller. Even if you create an instance of a collection inside a call, it would be wrong. In other words, you have to rethink your interface provided by your class, not just implementation.

Of course, not only listr could be null, but also std. You need to find it out by yourself. You cannot ask such questions every time such situation appears. 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 dereferences 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