Object reference not set to an instance of an object error

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In my UI layer, I have a Form with a tabControl, where a user control (with child controls) is placed. Upon loading user control, I get object reference not set to an instance of an object in if statement. My ID can be null, but how do I initialize it first? Or do the if statement differently? If there is no ID, my app generates a new ID when a new case is entered.

UI

C#

public CaseUserControl()
{
   InitializeComponent();
}

public void InitializeUserControl()
{
   //if there is an ID, load existing data.
   if (base.CaseData.ID != null)  //error here
   {
      this.LoadData();
   }
   else
   {
      //initialize new form
      this.InitNewForm();
   }
}

My BLL layer has

C#

//read only
public int? ID { get; set; }

C#

protected BaseCaseData(DataRow dr)
{
   ID = dr.Field<int>("ID", false);
}

My DAL code has an Insert SP and an Update SP, and a GetNewCaseNumber method.

Posted 13-Mar-15 10:47am

Sam 9100

Updated 13-Mar-15 11:25am

phil.o

v3

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Sergey Alexandrovich Kryukov 13-Mar-15 16:50pm

Who told you that ID is read-only?
—SA

2 solutions

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-03-13T10:54:00

Let's see.
In base.CaseData.ID, base cannot be null, but base.CaseData can. The fact that ID can also be null (as nullable) does not matter, because you did not reach this point. The exception is thrown when you dereference base.CaseData trying to access the ID property. If you try to dereference null, this exception is thrown.

That's all.

You really, really need to lean how to deal with such cases by yourself, as you cannot ask question every time it happens. 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

Sam 9100 · Answer 2 · 2015-03-31T06:14:00

Solution 2

All related issues revolved around not correctly creating the class instance in my form and in my user control. Once I set new instance and used that for all my child controls, it worked. This has been resolved.

Posted 31-Mar-15 6:14am

Sam 9100

Comments

Sergey Alexandrovich Kryukov 29-Apr-15 19:33pm

Sorry, this is not an answer. Such posts are considered as abuse. You could simply informs the readers about that in your question, using "Improve question". I would advise to remove this post, to avoid abuse reports.
—SA