Covariance of IChangeSet

[benjamin-hejl]

A question related to design. In my project I am using pattern like this for concrete implementations of an abstraction which manages other abstract items (e.g. IDeviceManager & IDevice):

SourceCache<SerialPortDevice, String> _cache = new ...;
...
IObservable<IChangeSet<IDevice, String>> WhenDevicesChanged() => _cache.Connect();

But this is not possible since IChangeSet<T> is not covariant (e.i. IChangeSet<out TObject, TKey>). I understand why. To make this possible, there would have to be some kind of IChange<out TObject, TKey>interface. Using the interface would mean allocations were made whenever changes occur, which does not happen now, since a struct Change<TObject,TKey> is used instead.

But when going through the code, I discovered, that within Change<TObject> for lists is a class. Is there some kind of reason for the different approach?

Are there some benchmarks which prove the performance drop is too big to enable this, in my opinion, logical use case?

I know I can handle my problem using the Cast operator and some other workarounds.

[RolandPheasant]

Alas the implementation of dd breaks the rules of covariance due to the signature of the changeset. It was less deigned in and more of a necessity of achieving the functionality