What is the empty Data entry on unhandled exceptions?

[ThomasArdal]

I'm implementing the UnhandledException event in WPF running on both .NET 6 and .NET 8:

AppDomain.CurrentDomain.UnhandledException += (sender, args) =>
    Log(args.ExceptionObject as Exception);

When throwing a new exception somewhere in my code, I get the UnhandledException event triggered as expected. But exceptions are decorated with an empty entry in the Data dictionary where the key is of type System.Object:

Do anyone know why this happens? Is it something that can be disabled or is this intended to include some additional information or something?

[miloush]

wpf/src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Threading/Dispatcher.cs

Lines 2755 to 2767 in ed058c1

	// see whether this dispatcher has already seen the exception.
	// This can happen when the dispatcher is re-entered via
	// PushFrame (or similar).
	if (!e.Data.Contains(ExceptionDataKey))
	{
	// first time we've seen this exception - add data to the exception
	e.Data.Add(ExceptionDataKey, null);
	}
	else
	{
	// we've seen this exception before - don't catch it
	return false;
	}

[ThomasArdal]

Thanks. So, it's actually some internal bookkeeping to see if an exception has already been handled? And it is (mis)using the Data dictionary for that?

[miloush]

Correct. I wouldn't necessarily call finding something you didn't put there yourself a misuse (it is writeable by any catch/finally on the stack and anybody else having a reference too), but I understand the mild annoyance.

[ThomasArdal]

It's not a problem that items are added to the dictionary. As you write, only an annoyance. Data items are indeed additional information about the exception. And this is somewhat an additional piece of information. It's just hard to know what information {object} is 😄

I personally prefer keeping items in Data as additional information about an exception that is interesting for anyone who gets hold of a reference to the exception. But that's probably a matter of taste. I can see how this can be used for bookkeeping too, I just think that I would have named it differently. But I'm happy knowing what this is now.

I'll remove it on my side. Thank you for the quick feedback.