Moq setup gives error System.NullReferenceException

Question

I am writing Unit tests for controller class using XUNIT. Here is my code.

private Mock<StudentService> _StudentService;
private StudentController StudentController;

[Fact]
public void GetStudentsBySubject_ShoutReturnNotNull()
{
    List<StudentViewModel> Students = new List<StudentViewModel>();
 _studentService = new Mock<StudentService>();
    string Subject = "Math";
    _StudentService.Setup(x => x.GetStudentsBySubject(Subject))
      .Returns((Students));

    var controller = new StudentController(_StudentService.Object);

    var result = controller.GetStudentsBySubject(Subject);

    Assert.IsType<List<StudentViewModel>>(result);            
}

It throws error at line.

_StudentService.Setup(x => x.GetCheapestStudentsBySubject(Subject))
      .Returns((Students));

Message: System.NullReferenceException : Object reference not set to an instance of an object

I tried many changes. But nothing worked. What could I be doing wrong here.

The following error appears after fixing the nullreference error:

System.NotSupportedException : Unsupported expression: x => x.GetStudentsBySubject(StudentControllerTests.<>c__DisplayClass2_0.Subject) Non-overridable members (here: StudentService.GetStudentsBySubject) may not be used in setup / verification expressions

Did you initialize "_StudentService" before use? (e.g. "_StudentService = new Mock()") — Sergey Vishnevskiy, Sep 19 '19 at 05:11
Regarding second problem: https://stackoverflow.com/questions/40828489/even-simple-moq-code-is-throwing-notsupportedexception — orhtej2, Sep 19 '19 at 06:56

Alex · Answer 1 · 2019-09-19T06:25:34.787

0

Your variable private Mock<StudentService> _StudentService; has not been initialized and is still null.

You need to assign it a value first:

_StudentService = new Mock<StudentService>();

Edit: Your follow-up error means that you need to mark the method GetStudentsBySubjectas virtual. Otherwise, Moq can't control the behavior of the method.

edited Sep 19 '19 at 06:25

answered Sep 19 '19 at 05:25

Alex

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I added but still gives error. Sorry that my code had some typing errors. I have corrected them. It gives the following errors.System.NotSupportedException : Unsupported expression: x => x.GetStudentsBySubject(StudentControllerTests.<>c__DisplayClass2_0.Subject) Non-overridable members (here: StudentService.GetStudentsBySubject) may not be used in setup / verification expressions. – Techie Sep 19 '19 at 06:05
@Techie see my follow up. add the `virtual` keyword to the GetStudentsBySubject method. – Alex Sep 19 '19 at 06:26
Can I fix it without changing the source method. – Techie Sep 19 '19 at 12:06
If you don't want to change the source method, you have to create an interface on StudentService and then mock the interface. Otherwise, no - the method has to be virtual. – Alex Sep 19 '19 at 12:17

Moq setup gives error System.NullReferenceException

1 Answers1