Question

I’m looking for some design advice for a new module of the application I am working on, particularly with regards to how to make the design testable.

The problem is very common – load some data from a database, run some operations against the data, and save the result to the database. Most other modules in the application have the following pattern:

private repo; //Set in constructor
public void Run() 
{
    Stuff stuff = repo.LoadStuff()
    Result result = RunOperationsInPrivateMethod(stuff); //private method
    repo.SaveResult(result);
}

So to test this, I see that I have a couple of choices:

1. Inject a mock repo that I can use to return a Stuff and verify a Result.
2. Refactor RunOperationsInPrivateMethod to protected access, and test
   the operations directly.

Am I missing any other options? What are peoples preferences?

Answer 1

In general, don’t test private methods, instead, think whether your private method really should be a public method of another class. ie, decompose your object into smaller objects with focused functionality.

eg, perhaps Run should be

private repo; //Set in constructor
private IOperation operation; // injected in constructor or through dependency injection.
public void Run() 
{
    Stuff stuff = repo.LoadStuff()
    Result result = operation.Run(stuff); //private instance with public method
    repo.SaveResult(result);
}

then Run would be a public method of an operations class

class SecretOperation : IOperation
{
   public void Run(Stuff stuff) { /* secret stuff */ }
}

Then also, you wouldn’t have to load a Stuff from a database to test, just create a stuff in a fixture focused on testing SecretOperation. Now your unit tests can be more focused.