Fix an InvalidOperationException when WeakEventTable.Purge is called …

[kmaddock]

…with EnableWeakEventMemoryImprovements enabled

Fixes #6452

Description

If EnableWeakEventMemoryImprovements is true, the WeakEventTable.Purge enumerates through _dataTable calling WeakEventManager.Purge on each list:

IDictionaryEnumerator ide = _dataTable.GetEnumerator() as IDictionaryEnumerator;
while (ide.MoveNext())
{
    ...
    foundDirt |= key.Manager.PurgeInternal(source, ide.Value, purgeAll);
    ...
}

WeakEventManager.PurgeInternal redirects to .Purge, which will check the ListenerList for concurrent readers, and clone and update Table (_dataTable) if concurrent readers exist:

if (ListenerList.PrepareForWriting(ref list) && source != null)
{
    Table[this, source] = list;
}

Writing to _dataTable invalidates the enumerator in WeakEventTable, which will throw an InvalidOperationException.

This PR exits WeakEventManager.Purge if EnableWeakEventMemoryImprovements is true, the list purge will be retried next time WeakEventTable is purged,

Customer Impact

Our App throws InvalidOperationException and crashes.

Regression

No

Testing

I reprod with the test case attached to issue #6452 and confirmed that this PR fixes it.

Risk

It seems pretty low risk to me

Microsoft Reviewers: Open in CodeFlow