public static void Main()
{
// This code shows that both service implementations have access to the database and
// that they still fulfill the same interface contract
IReportingService untestableReportingService = new Problem.ReportingService();
Console.WriteLine(
"Students enrolled, queried via the untestable service implementation: {0}",
untestableReportingService.GetNumberOfEnrolledStudents());
IReportingService testableReportingService = new Solution.ReportingService();
Console.WriteLine(
"Students enrolled, queried via the testable service implementation: {0}",
testableReportingService.GetNumberOfEnrolledStudents());
Console.ReadKey();
}
public void AlasHowDoYouTestThisWithoutAWorkingDatabaseConnection()
{
// Arrange
// Step one:
// Arrange the test data the linq statements within the ReportingService instance should run against.
var studentWithEnrollment = new Student
{
Enrollments = new List<Enrollment>
{
new Enrollment()
}
};
var studentWithoutEnrollment = new Student
{
Enrollments = new List<Enrollment>()
};
var testStudents = new List<Student>
{
studentWithEnrollment,
studentWithoutEnrollment
}.AsQueryable(); // Step two: Make the test data available as an IQueryable
// or in other words: As the type that linq runs its queries against.
// At this point we can already predict how many students should be counted as enrolled
// -> We put one student with enrollments and one student without enrollments into our set of test data,
// so once we hook the ReportingService up with the test data, the answer to GetNumberOfEnrolledStudents()
// should be 1.
var expectedNumberOfEnrolledStudents = 1;
// Step three:
// Create a mock DbSet
var mockStudentsDbSet = new Mock<DbSet<Student>>();
// A DbSet implements IQueryable to allow the execution of linq queries against the DbSet itself.
// That's good news, because that means that we can just hook up our selfmade IQueryable 'students'
// to our mocked DbSet, which means that any time a linq query is executed against our mocked DbSet it really
// gets executed against our selfmade IQueryable 'students' which - as you might remember - contains exactly
// the test data we want it to contain and does certainly not require any database connection whatsoever.
//
// The code to hook up our IQueryable to the mocked DbSet is kinda ugly though...
mockStudentsDbSet.As<IQueryable<Student>>().Setup(queryable => queryable.Provider).Returns(testStudents.Provider);
mockStudentsDbSet.As<IQueryable<Student>>().Setup(queryable => queryable.Expression).Returns(testStudents.Expression);
mockStudentsDbSet.As<IQueryable<Student>>().Setup(queryable => queryable.ElementType).Returns(testStudents.ElementType);
mockStudentsDbSet.As<IQueryable<Student>>().Setup(queryable => queryable.GetEnumerator()).Returns(testStudents.GetEnumerator);
// Step four:
// Next we need to mock the DbContext or in our case the SchoolContext. We can't simply replace the Students
// DbSet in an original SchoolContext because the DbContext part would still try to open a connection to the
// database - which our unit test does not have access to.
var mockSchoolContext = new Mock<SchoolContext>();
// Step five:
// Hook out mocked students DbSet into out mocked SchoolContext so any call against mockSchoolContext.Students
// gets redirected to our mocked students DbSet.
mockSchoolContext.Setup(db => db.Students).Returns(mockStudentsDbSet.Object);
// Important: The DbSet-Properties within the DbContext / SchoolContext need to be declared as
// virtual or Moq will be unable to mock the properties.
// i.e.
// public virtual DbSet<Student> { get; set; }
// instead of
// public DbSet<Student> { get; set; }
// Database first entity framework automatically marks the properties as virtual.
// If you use the entity framework code first approach you have to ensure this yourself.
// Step six:
// Now we need to hook up our ReportingService with our mocked SchoolContext. This can be achieved via
// dependency injection. This example uses a simple form of constructor injection. Depending on your
// situation a more complex solution might be required.
IReportingService reportingService = new ReportingService(mockSchoolContext.Object);
// And that's it! We're done!
// The following code now executes against our test IQueryable and returns the result predicted
// above. Through mocking and decoupling we are now able to test any linq statements written against
// entity framework while completely ignoring the underlying database.
// Act
var actualNumberOfEnrolledStudents = reportingService.GetNumberOfEnrolledStudents();
// Assert
Assert.AreEqual(expectedNumberOfEnrolledStudents, actualNumberOfEnrolledStudents);
}
