System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
I am adding/removing from an ObservableCollection
You can use a ObservableConcurrentCollection class. They are in a package provided by Microsoft in the Parallel Extensions Extras library.
You can get it prebuilt by the community on Nuget: https://www.nuget.org/packages/ParallelExtensionsExtras/
Or get it from Microsoft here:
https://code.msdn.microsoft.com/ParExtSamples