Unit Testing a Class With A Private Constructor

Yes you can using reflexion. your code is neraly there;

you can get properties and fields of the types with typeof(Course).GetProperty("PropertyName") then you can use SetValue to set the desired value, and pass as parameter first the instance to modify then the value. in your case true;

note: in your example you will need to add the Collection of students too, if your Open is true.

Here there is a working example:

[Fact]
public void CanRegisterStudentForOpenClass()
{
    var course = (Course)Activator.CreateInstance(typeof(Course), true);

    typeof(Course).GetProperty("Open").SetValue(course, true, null);
    ICollection<Student> students = new List<Student>();
    typeof(Course).GetProperty("Students").SetValue(course, students, null);

    course.Register("Bob");

    Assert.Single(course.Students);
}

If you would rather not use reflection, then I recommend you use internal classes (instead of private) and using the InternalsVisibleToAttribute on your implementation assembly.

You can find more about the attribute here. Here's a quick guide on how you can use it!

Step 1. Add this attribute to your assembly that wants its internal code tested.

[assembly: InternalsVisibleToAttribute("MyUnitTestedProject.UnitTests")]

Step 2. Change private to internal.

public class Course
{
    internal Course()
    {
    }

    public int Id { get; internal set; }
    public string Name { get; internal set; }
    public bool Open { get; internal set; }
    public virtual ICollection<Student> Students { get; internal set; }

    /* ... */
}

Step 3. Write your tests like normal!

[Fact]
public void CanRegisterStudentForOpenClass()
{
    var course = new Course();
    course.Id = "#####";
    course.Register("Bob");
}

As a few people have mentioned here, unit testing something private is either a code smell, or a sign you're writing the wrong tests.

In this case, what you would want to do is use EF's in-memory database if you're using Core, or mocking with EF6.

For EF6 You can follow the docs here

I would say rather than newing your dbContext where you do, pass it in via Dependency Injection. If that's beyond the scope of the work you're doing, (I'm assuming this is actual coursework, so going to DI may be overkill) then you can create a wrapper class that takes a dbcontext and use that in place.

Taking a few liberties with where this code is called from...

class Semester
{
  //...skipping members etc

  //if your original is like this
  public RegisterCourses(Student student)
  {
    using (var db = new SchoolContext())
    {
      RegisterCourses(student, db);
    }
  }

  //change it to this
  public RegisterCourses(Student student, SchoolContext db)
  {
    var course = db.Courses.Include(x => x.Students).Where(x => x.Name == courseName).First();

      course.Register(studentName);
      db.SaveChanges();
  }
}  

[Fact]
public void CanRegisterStudentForOpenClass()
{
  //following after https://docs.microsoft.com/en-us/ef/ef6/fundamentals/testing/mocking#testing-query-scenarios
  var mockCourseSet = new Mock<DbSet<Course>>();
  mockCourseSet.As<IQueryable<Course>>().Setup(m => m.Provider).Returns(data.Provider);
  mockCourseSet.As<IQueryable<Course>>().Setup(m => m.Expression).Returns(data.Expression);
  mockCourseSet.As<IQueryable<Course>>().Setup(m => m.ElementType).Returns(data.ElementType);
  mockCourseSet.As<IQueryable<Course>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());
  //create an aditional mock for the Student dbset
  mockStudentSet.As.........

  var mockContext = new Mock<SchoolContext>();
  mockContext.Setup(c => c.Courses).Returns(mockCourseSet.Object);
  //same for student so we can include it

  mockContext.Include(It.IsAny<string>()).Returns(mockStudentSet); //you can change the isAny here to check for Bob or such

  
  var student = Institution.GetStudent("Bob");
  var semester = Institution.GetSemester(Semester.One);
  semester.RegisterCourses(student, mockContext);
}

If you're using EFCore you can follow it along from here

You can fake private constructors and members using TypeMock Isolator or JustMock (both paid) or using MS Fakes (only available in VS Enterprise).

There is also a free Pose library that allows you to fake access to properties.

Unfortunately, the private constructor can't be forged. Therefore, you will need to create an instance of the class using reflection.

Add package.
Open namespace:

using Pose;

Test code:

[Fact]
public void CanRegisterStudentForOpenClass()
{
    var course = (Course)Activator.CreateInstance(typeof(Course), true);

    ICollection<Student> students = new List<Student>();
    Shim studentsPropShim = Shim.Replace(() => Is.A<Course>().Students)
                                .With((Course _) => students);

    Shim openPropShim = Shim.Replace(() => Is.A<Course>().Open)
                            .With((Course _) => true);

    int actual = 0;

    PoseContext.Isolate(() =>
    {
        course.Register("Bob");
        actual = course.Students.Count;
    },
    studentsPropShim, openPropShim);

    Assert.Equal(1, actual);
}

You can create a JSON representation of your default instance and deserialize it with Newtonsoft.

Something like this:

using System.Reflection;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;
using privateConstructor;

namespace privateConstructorTest
{
    [TestClass]
    public class CourseTest
    {
        [TestMethod]
        public void Register_WhenOpenIsTrue_EnableAddStudents()
        {
            // Arrange
            const string json = @"{'Id': 1, 'name':'My Course', 'open':'true', 'students':[]}";
            var course = CreateInstance<Course>(json);

            // Act
            course.Register("Bob");

            // Assert
            Assert.AreEqual(1, course.Students.Count);
        }

        [TestMethod]
        public void Register_WhenOpenIsFalse_DisableAddStudents()
        {
            // Arrange
            const string json = @"{'Id': 1, 'name':'My Course', 'open':'false', 'students':[]}";
            var course = CreateInstance<Course>(json);

            // Act
            course.Register("Bob");

            // Assert
            Assert.AreEqual(0, course.Students.Count);
        }

        private static T CreateInstance<T>(string json) =>
            JsonConvert.DeserializeObject<T>(json, new JsonSerializerSettings
            {
                ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor,
                ContractResolver = new ContractResolverWithPrivates()
            });

        public class ContractResolverWithPrivates : CamelCasePropertyNamesContractResolver
        {
            protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
            {
                var prop = base.CreateProperty(member, memberSerialization);
                if (prop.Writable) return prop;
                var property = member as PropertyInfo;
                if (property == null) return prop;
                var hasPrivateSetter = property.GetSetMethod(true) != null;
                prop.Writable = hasPrivateSetter;

                return prop;
            }
        }
    }
}

In order to have a cleaner test class, you can extract the JSON strings and the helper code that creates the instance.