Can you rethrow a c# exception on a different thread?

Question

Is it legal and safe in C# to catch an exception on one thread, and then re-throw it on another.

E.g. is this legal

Exception localEx = null;

Thread mythread = new Thread() { () =>
                   {
                        try
                        {
                            DoSomeStuff();
                        }
                        catch(Exception ex)
                        {
                            localEx = ex;
                        }
                    });

myThread.Start();
...
myThread.Join();

if(localEx != null)
   throw localEx;    // rethrow on the main thread

I think it is legal, but I'm having trouble finding any doco that proves it. The closest I found was a brief mention of transferring exceptions between threads here: http://msdn.microsoft.com/en-us/library/ms229005.aspx

Answer 1

Yes, it's legal. Exceptions are (generally speaking) descriptive objects with no thread affinity.

You'd be better off wrapping your thread exception in a new exception:

throw new Exception("Something descriptive here", localEx);

That way, the stack trace in localEx will be preserved (as the InnerException of the new exception).

Answer 2

I don't see why it wouldn't work, but you need to remember that you aren't actually rethrowing the exception. You are throwing a new exception, that just happens to be the same exception object. So, for example, the stack trace will say it was thrown from "throw localEx;" instead of wherever the original exception came from.

Answer 3

What you're doing is not a rethrow. It's a new throw of an exception instance you happened to have in a variable. Even if you were using only a single thread, this would be a bad idea, as it makes the exception look like it came from the "throw" site. With multiple threads, I have no idea how anyone would figure out there had been a thread change.

Answer 4

It is legal and it isn't a rethrow, it's a new exception being thrown on another thread (with the same exception object)

Answer 5

Absolutely. System.AggregateException is added to .NET 4 for specifically that purpose during parallel operations.

Answer 6

I don't know why you think its not legal. If it were illegal surely the compiler would catch or the runtime would throw an exception. As a matter of fact I use this pattern. @John In a windows forms app that calls web services using a background thread I use this way. The exception is then handled in the Application.ThreadException top level handler, logged etc. It is unnecessary to know in which thread the exception occured.