InSequence

Okay, so I have some code which is of the format

do 
{
   while(reader.Read())
   {
      // do something 
   }
} while (reader.ReadNextPage())

the basic premise is Read some data from somewhere until the data is exhausted, then read the next page of data and so on, until no data is left to read.

I wanted to unit test aspects of this by mocking out the reader and allowing me to isolate the specific functionality within the method. Ofcourse I could have refactored this method to test just the inner parts of the loop, but this is not always desirable as it still means the looping expectation is not unit tested.

I can easily mock the ReadNextPage to return false to just test one pages of data, but the Read method itself needs to return true initially, but also must return false at some point or the unit test will potentially get stuck in an infinite loop. Hence, I need to be able to eventually return false on the Read method.

Using InSequence, we can return different values on the calls to the Read method, for example using

Mock.Arrange(() => reader.ReadNextPage()).Returns(false);
Mock.Arrange(() => reader.Read()).Returns(true).InSequence();
Mock.Arrange(() => reader.Read()).Returns(false).InSequence();

Here the first call to Read obviously returns true, the next call returns false, so the unit test will actually complete and we’ll successfully test the loop and whatever is within it.