Csharp List.Contains) 不能在设置验证表达式中使用

我想模拟一个方法，但我的方法是在条件下，我需要的值是真的，这个操作使我的测试通过，如果我不模拟我的方法，它给我空异常，所以这很重要

  [Test]
    public void Can_return_price_according_to_semester_status()
    {
        var product = new Product
        {
            ProductTypeId = 15,
            Id = 1,
            Name = "Product name 1",
            Price = 12.34M,
            CustomerEntersPrice = false,
            Published = true,
            PreRegistrationPrice = 10.99M
        };

        _productService.Setup(x => x.GetSemesterProductTypeIds(It.IsAny<int[]>())
        .Contains(product.ProductTypeId)).Returns(true);// this is my target mock



        var customer = new Customer();
        _priceCalcService.GetFinalPrice(product, customer, 0, false, 1).ShouldEqual(product.Price);
        _priceCalcService.GetFinalPrice(product, customer, 0, false, 1).ShouldEqual(product.PreRegistrationPrice);
    }

和GetFinalPrice:

    if (_productService.GetSemesterProductTypeIds().Contains(product.ProductTypeId))// this is target of my mock
            {
                if (enrollmentTypeId <= 0)
                {
                    if (product.SemesterStatus == SemesterStatus.PreRegistration)
                    {
                        enrollmentTypeId = (int)EnrollmentType.PreRegistered;
                    }
                }
                if (enrollmentTypeId == (int)EnrollmentType.PreRegistered)
                {
                    price = product.PreRegistrationPrice;
                }
            }// some operation after this

problem