EF Core InMemory数据库初始化在单元测试用例中抛出System.NullReferenceException异常

Question

我正在尝试使用内存数据库来测试我的efcore项目和followed this article。

以下是我的设置：

    public class Student
        {

            public int Id { get; set; }

            public string FirstName { get; set; }

            public string LastName { get; set; }

         }


    public class StudentContext : DbContext
    {
       public StudentContext(){
       }
        public StudentContext(DbContextOptions options)
            : base(options)
        {
        }

        public DbSet<Student> Student { get; set; }
    }


 [TestClass]
    public class StudentTest
    {
        private StudentContext _dbContext;

        [TestInitialize]
        public void Setup()
        {
            var optionBuilder = new DbContextOptionsBuilder<StudentContext>();
            var options = optionBuilder.UseInMemoryDatabase().Options;
            _dbContext = new StudentContext (options);
            _dbContext.Users.Add(new Student
            {
                Id = 1,
                FirstName = "tester",
                LastName = "tester",
            });
            _dbContext.SaveChanges();
        }

        [TestMethod]
        public void ValidateStudent()
        {
            var service = new StudentService(_dbContext);
            var response = service.AddStudent(new Student{
                Id = 2,
                FirstName = "tester2",
                LastName = "tester2",
            });
            Assert.IsTrue(response);
        }


        [TestCleanup]
        public void Cleanup()
        {
            _dbContext.Database.EnsureDeleted();
            _dbContext = null;
        }
    }

它在以下行抛出System.NullReferenceException：

var optionBuilder = new DbContextOptionsBuilder<StudentContext>();

它在stacktrace中显示的唯一原因如下：

threw exception. System.NullReferenceException: System.NullReferenceException: Object reference not set to an instance of an object..
    at StudentContext.OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        at Microsoft.EntityFrameworkCore.DbContext.InitializeServices()
       at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
       at Microsoft.EntityFrameworkCore.DbContext.get_StateManager()
       at Microsoft.EntityFrameworkCore.DbContext.EntryWithoutDetectChanges[TEntity](TEntity entity)
       at Microsoft.EntityFrameworkCore.DbContext.SetEntityState[TEntity](TEntity entity, EntityState entityState)
       at Microsoft.EntityFrameworkCore.DbContext.Add[TEntity](TEntity entity)
       at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.Add(TEntity entity)

我无法找到此错误的原因。