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When you are searching for a bug, you do need to understand the implementation. Of course by definition something is "wrong". Calling a function is fine by the author since you can easily jump to its definition. But if the actual function call is determined at run-time based on XML configuration, then you first need to figure out how that fits together for the particular case you need to investigate.

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OP here. You both are right. I'm not saying that DI and interfaces are bad. Of course not. I'm just trying to find a balance between modularity, loose coupling and code graph connectivity. Of course it's not just black or white.

In a case like this, you really ought to have a standard set of unit tests that covers the contract of the interface. If you run that against all the implementations and none of the tests fail, the bug just isn't in the implementation itself and you can safely move on to looking for integration issues. Of course, if the unit tests for the interface are incomplete you can't rely on them, but in that case you've built yourself a leaky abstraction that you should go back and look at before doing any more systems testing. Effective dependency injection simply requires comprehensive unit testing.

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