# 2.1 Three kinds of test

| Test | What it builds | What it catches |
|---|---|---|
| Unit | One junction, with its dependencies passed in by hand | A junction that computes the wrong thing |
| Train | The train, through the same `AddTrax` registration as `Program.cs` | A chain that does not run as a whole, a run that is not recorded |
| Host | The whole application, in memory, called over HTTP | A `Program.cs` that no longer starts, an endpoint that answers wrongly |

Most tests are unit tests: they are fast and say exactly what broke. A few train and host tests
prove the pieces still fit.

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Next: [2.2 Add a solution and a test project](/docs/book/testing-place-order/add-a-solution-and-a-test-project)