Question

Lets say I have the below method called DoSomething.

When writing unit tests how do I know whether to use a Fake, Mock or Override approach and why is one better than the other?

public List<MyClass> DoSomething()
{
  List<MyClass> data = GetData();

  if (data.Count == 0)
    return new List<MyClass>();

  data = GetFormattedData(data);

  if (data.Count == 0)
    return new List<MyClass>();

  return data;
}

[Test]
public void DoSomething_NoData_ReturnsEmptyList()
{
  //Change method parameters to pass in IDataProvider that exposes GetData method
  //Create FakeProvider class implementing IDataProvider
  //Ensure FakeProvider.GetData returns no data

  //Create FakeClass that inherits class from DoSomething class
  //Make FakeClass.GetData return no data
  //When DoSomething is called in the test it will call the parent class

  //Create Mock of class that DoSomething/GetData/GetFormattedData is in
  //Tell mock to make sure GetData returns empty list
  //Call DoSomething in test
}

[Test]
public void DoSomething_NoFormattedData_ReturnsEmptyList()
{
   //Same possibilities exist as above
}

Answer 1

In this case you could supply your function with the data so that the function signature would be public List<MyClass> DoSomething(List<MyClass data) Then your function would only have a single responsibility and that would be to format the data.

If you still want to do the data fetch withing your function and you will be accessing a database or external service I would use dependency injection and mocking to test the function.

Generally it is good to avoid dependencies if possible. It all depends of course, if you don’t do any other data fetching in your class then it’s just a matter of how far down you will send your dependency.