NullReferenceException

[Guest]

I have a multi-threaded VB application and occasionly I get a
'System.NullReferenceException' on a call to
System.Windows.Forms.Control.Refresh. Why is this?

tks
Mark

 

[Mattias Sjögren]

I have a multi-threaded VB application and occasionly I get a
'System.NullReferenceException' on a call to
System.Windows.Forms.Control.Refresh. Why is this?

Are you calling Refresh from a background thread? In general you
should only manipulate UI objects on the thread that created them.


Mattias

 

[Herfried K. Wagner [MVP]]

I have a multi-threaded VB application and occasionly I get a
'System.NullReferenceException' on a call to
System.Windows.Forms.Control.Refresh. Why is this?

Some additional information on Windows Forms and threads can be found here:

Multithreading in Windows Forms applications
<URL:http://dotnet.mvps.org/dotnet/faqs/?id=multithreading&lang=en>

 

[Guest]

I'm calling Refresh from the UI thread...though there is some UI manipulation
from a background thread.

Mark.

 

[Oenone]

Are you calling Refresh from a background thread? In general you
should only manipulate UI objects on the thread that created them.

Incidentally, did the rules in this area change in Visual Studio 2005? I'm
sure I read somewhere that they would modify the .NET framework so that
manipulating WinForms UI controls from other threads would no longer cause
any problems, but perhaps I imagined it..?

 

[Greg Burns]

Are you thinking of the new BackgroundWorker control?

http://msdn2.microsoft.com/en-us/library/system.componentmodel.backgroundworker

Greg

 

[Brian Gideon]

Well, an exception is now thrown immediately informing you that an
attempt to access a control on a thread other than the one that created
it occurred. Updating controls on other threads always has and is
likely to always cause problems. 2.0 is just kind enough to let you
know about the problem up front. Controls are still not thread-safe.

Brian

 

[Oenone]

Are you thinking of the new BackgroundWorker control?

http://msdn2.microsoft.com/en-us/library/system.componentmodel.backgroundworker

I wasn't, but I'll investigate this class (which I hadn't previously
noticed!)

 

[Oenone]

2.0 is just kind enough to
let you know about the problem up front. Controls are still not
thread-safe.

Ah well, that's still an improvement I guess. The thing that always worried
me in v1.1 was that UI updates usually worked, but I was always aware that
at some point they would fail spectacularly and unpredictable. At least it's
predictable this way.

Thanks,