NullReferenceException - but where?!

[musosdev]

Hi peeps,

I'm suddenly getting a very strange error on one of my websites. It seems to
be crashing within SaveViewState (?) but I can't tell if that the case, or if
there is something else going on.

The error occurs when I load a certain page with 2 lists, one shows a
collection of contacts, and the other looks at which of those contacts are
tied to a project. On load, I load the contacts list, and then loop through
the project contacts and copy the data from the other list. All contacts in
the project are still in the contacts list, so thats ok, but why am I getting
a viewstate crash?!

Here's the error...

Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information about
the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set
to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the current web
request. Information regarding the origin and location of the exception can
be identified using the exception stack trace below.

Stack Trace:
[NullReferenceException: Object reference not set to an instance of an
object.]
System.Web.UI.WebControls.ListItemCollection.SaveV iewState() +102

System.Web.UI.WebControls.ListItemCollection.Syste m.Web.UI.IStateManager.SaveViewState() +4
System.Web.UI.HtmlControls.HtmlSelect.SaveViewStat e() +39
System.Web.UI.Control.SaveViewStateRecursive() +59
System.Web.UI.Control.SaveViewStateRecursive() +160
System.Web.UI.Control.SaveViewStateRecursive() +160
System.Web.UI.Page.SaveAllState() +466
System

What's going on!?

Thanks,
Dan

[Walter Wang [MSFT]]

Hi Dan,

Glad to see you again in ASP.NET newsgroup! Have you seen my reply in your
post "getting the current page from inside a web control"? Please feel free
to reply there if there's anything I can help.

Regarding this post, I understand that you're seeing some
NullReferenceException and want to know how to debug it effectively.

From the stack trace, the exception is thrown from
ListItemCollection.SaveViewState, which is simply saving each ListItem into
ViewState.

If this exception can be reproduced on your develop machine, please use
following steps to debug it:

1. In Visual Studio 2005, click menu "Debug/Exceptions..."
2. Click button "Find", enter "System.NullReferenceException", click "OK"
3. When the exception if found in the list, check it, click "OK" to close
the exceptions dialog.
4. Start debugging your web site, when the exception is thrown, Visual
Studio will break at the code where the exception is thrown, since we don't
have the source code of System.Web.dll, we simply click the button "Show
Disassembly" when prompted.
5. Click on menu "Debug/Windows/Call Stack" to show the call stack, click
on menu "Debug/Windows/Locals" to show the local variables window;
6. In the call stack window, double click on the top most stack frame,
inspect the Locals window to view the variables; you should see "this" with
System.Web.UI.WebControls.ListItemCollection type, try to inspect its
state. I think some of its listItems might be null.
If this issue can be easily reproduced with a smaller web site; you can
send it to me and I will help to find the root cause. Thanks.
Sincerely,
Walter Wang (wa****@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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