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Questions:
InnerHandler can be null

InnerHandler cannot be null if DefaultHttpClientFactory is used.

Code links below.

ExpiredHandlerTrackingEntry.InnerHandler is assigned only in .ctor from ActiveHandlerTrackingEntry.Handler.InnerHandler.

ActiveHandlerTrackingEntry.Handler is assigned only in .ctor, and ActiveHandlerTrackingEntry is created only in one place where LifetimeTrackingHttpMessageHandler is also explicitly created.

LifetimeTrackingHttpMessageHandler.InnerHandler is always assigned in .ctor (it cannot be unassigned).

LifetimeTrackingHttpMessageHandler is derived from DelegatingHandler. DelegatingHandler.InnerHandler disallows null on assignment. So even if null is somehow passed to LifetimeTrackingHttpMessageHandler's .ctor -- it might be achievable by substituting HttpMessageHandlerBuilder in DI and making Build() return null -- DelegatingHandler's .ctor will throw.