Using IoC for Unit Testing

Question

How can a IoC Container be used for unit testing? Is it useful to manage mocks in a huge solution (50+ projects) using IoC? Any experiences? Any C# libraries that work well for using it in unit tests?

Accepted Answer

Generally speaking, a DI Container should not be necessary for unit testing because unit testing is all about separating responsibilities.

Consider a class that uses Constructor Injection

```
public MyClass(IMyDependency dep) { }
```

In your entire application, it may be that there's a huge dependency graph hidden behind `IMyDependency`, but in a unit test, you flatten it all down to a single Test Double.

You can use dynamic mocks like Moq or RhinoMocks to generate the Test Double, but it is not required.

```
var dep = new Mock<IMyDependency>().Object;
var sut = new MyClass(dep);
```

In some cases, an auto-mocking container can be nice to have, but you don't need to use the same DI Container that the production application uses.

Using IoC for Unit Testing

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