Messenger is throwing an exception because the collection of subscribers has been modified.

[smithmx]

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.Collections.Generic.List1.Enumerator.MoveNextRare() at Bifrost.Messaging.Messenger.Publish[T](T content) at Drofus.Components.UI.Views.SystemComponentsViewModel.Refresh(Object sender) --- End of inner exception stack trace --- at mscorlib!0x06001205!System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters) at Bifrost.Execution.WeakDelegate.DynamicInvoke(Object[] arguments) at Bifrost.Messaging.Messenger.Publish[T](T content) at Nosyko.Drofus.Revit.DrofusRevitPlugin.ProcessTask(UIApplication uiapp, Queue1 tasks)

Instead of

foreach (var subscriber in _subscribers[type])

we should do

foreach (var subscriber in _subscribers[type].ToList())

[einari]

Would it be an option to make the _subscribers a ConcurrentBag instead.

[smithmx]

Yeah, that's fine as well. It might even be better as we are more likely removing subscriptions while iterating than adding them.