Document mixing real framework services with AutoMock (#49)

[tillig]

Addresses #49.

Background

In #49, constructing an EF Core DbContext inside an AutoMock container that also had logging wired up threw a NullReferenceException deep inside Microsoft.Extensions.Logging.

Root cause (not a defect to "fix" in code)

AutoMock supplies a mock for any service it can't otherwise resolve, including the individual elements of an otherwise-empty collection dependency. So IEnumerable<ILoggerProvider> — which is legitimately empty in normal DI — instead gets a single mock element injected. That loose mock's CreateLogger returns null, and LoggerFactory then dereferences the null.

This single-element injection is intentional, long-standing behavior (it's what lets you test collection-style dependencies), and it's shared identically with the FakeItEasy integration. It is also non-recursive — it stops at the mock boundary and does not cascade into nested collections. So rather than change that behavior (a breaking change with real downsides), this documents the interaction and the workaround.

The test-ordering quirk in the original report is EF Core's static service-provider cache masking the problem when a "good" path runs first; it's outside our control.

Change

Adds <remarks> to the AutoMock class describing that adding real framework services into the container can yield null-returning mocks in framework-consumed collections, and how to avoid it (register the concrete services the framework consumes).

Related

Companion documentation PR with the detailed guidance: autofac/Documentation#185

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.99%. Comparing base (595df74) to head (d4363f6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #61   +/-   ##
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  Coverage    90.99%   90.99%           
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  Files            3        3           
  Lines          111      111           
  Branches        21       21           
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  Hits           101      101           
  Misses           6        6           
  Partials         4        4           

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