Should I care about the number of calls to a mocked object - unit-testing

Question

The title is self explanatory but I'll elaborate.
I'm using FakeItEasy framework where I have the option to check the number of times a method have been invoked.
A.CallTo(() => foo.Bar()).MustHaveHappened();
But by doing this I'm testing the internal behavior of my code. I'm not testing a returned value or a state change which is what a good test should do.
so my question is this: is it a good practice to test the number of times a method have been called?

Answer 1

You should care about the number of times a dependency was invoked in many cases.
For example, suppose your have a service that inserts some data into some database by invoking a web service.
public interface IDataInserter
{
void Insert(Data[] data);
}
And assume that in some cases you need to insert some 10000 items. But the web service cannot handle such volume of data in a single call. So you decide to create a decorator that will split the data into multiple chunks and send each chunk in a single request.
public class SplittingDecorator : IDataInserter
{
private readonly IDataInserter m_DataInserter;
public SplittingDecorator(IDataInserter data_inserter)
{
m_DataInserter = data_inserter;
}
public void Insert(Data[] data)
{
var chunks =
data
.Select((d, i) => new {d, i})
.GroupBy(x => x.i/50)
.Select(x => x.Select(y => y.d).ToArray())
.ToList();
foreach (var chunk in chunks)
{
m_DataInserter.Insert(chunk);
}
}
}
When you want to test the SplittingDecorator class, you will create a mock for the data_inserter constructor argument.
In such test, you need to assert that the mocked IDataInserter was invoked some X times when you invoke SplittingDecorator.Insert with a data of size Y.
For example, if the data size (length of the data array) is 160 items, you want to check that the mock was invoked 4 times.