如何对 ViewModel 使用方法进行单元测试？

Question

我想测试一个以 ViewModel 作为参数的辅助方法。我遇到的问题是测试似乎需要我实例化并分配我的 ViewModel 使用的所有模型。在我下面给出的示例中，这没什么大不了的，因为只有几个，但在我正在处理的实际 VM 中却有很多。有没有其他方法可以做到这一点，这样我就不必创建和分配每个对象？

用于说明目的的示例代码...

模型

public class Meal
{
    public int MealID { get; set; }

    public string Title { get; set; }
    public decimal Cost { get; set; }
}

public class Beverage
{
    public int BeverageID { get; set; }

    public string Title { get; set; }
    public decimal Cost { get; set; }
}

public class Desert
{
    public int DesertID { get; set; }

    public string Title { get; set; }
    public decimal Cost { get; set; }
}

视图模型

public class DinnerViewModel
{
    public Meal Meal { get; set; }
    public Beverage Beverage { get; set; }
    public Desert Desert { get; set; }

    public decimal SalesTax { get; set; }
    public bool SeniorDiscount { get; set; }
}

帮手

public class Calculator
{
    public decimal Total(DinnerViewModel dvm)
    {
        decimal subtotal = dvm.Meal.Cost + dvm.Beverage.Cost + dvm.Desert.Cost;

        if (dvm.SeniorDiscount)
        {
            subtotal = subtotal - (subtotal * 0.1M);
        }

        return subtotal + (subtotal * dvm.SalesTax);
    }
}

测试

[TestMethod]
public void CalculatorReturnsCorrectTotalForNonSenior()
{
    DinnerViewModel dvm = new DinnerViewModel();
    dvm.Meal.Cost = 7M;
    dvm.Beverage.Cost = 1M;
    dvm.Desert.Cost = 2M;
    dvm.SalesTax = 0.08M;
    dvm.SeniorDiscount = false;

    Calculator calc = new Calculator();

    decimal expected = 10.80M;
    decimal actual = calc.Total(dvm);

    Assert.AreEqual(expected, actual, "The actual value does not match the expected value.");
}

这会导致“NullReferenceException”错误。正如我所说，我可以创建和分配必要的对象...

[...]
Meal meal = new Meal();
dvm.Meal = meal;
dvm.Meal.Cost = 7M;
[...]

...一旦全部完成，测试就会成功——但这似乎需要在更大的 VM 上进行大量工作，我觉得我必须做一些事情来让这更容易。

Answer 1

您会看到 NullReferenceException，因为 Meal、Beverage 和 Desert 实例在您实例化它时没有分配给 DinnerViewModel 的相应属性。所以，所有这些属性都是null。您应该在访问这些属性之前创建并分配新对象。您可以使用视图模型的构造函数：

public class DinnerViewModel
{
    public DinnerViewModel()
    {
        Meal = new Meal();
        Beverage = new Beverage();
        Desert = new Desert();
    }

    public Meal Meal { get; set; }
    public Beverage Beverage { get; set; }
    public Desert Desert { get; set; }

    public decimal SalesTax { get; set; }
    public bool SeniorDiscount { get; set; }
}

我还喜欢创建一些返回存根进行测试的辅助方法。它消除了重复并使您的测试清晰：

private DinnerViewModel CreateTenDollarsDinner()
{
    return new DinnerViewModel {
        Meal = new Meal { Cost = 7M },
        Beverage = new Beverage { Cost = 1M },
        Desert = new Desert { Cost = 2M },
        SalesTax = 0.08M,
        SeniorDiscount = false
    };
}

[TestMethod]
public void CalculatorReturnsCorrectTotalForNonSenior()
{
    DinnerViewModel dvm = CreateTenDollarsDinner();    
    Calculator calc = new Calculator();
    Assert.AreEqual(10.80M, calc.Total(dvm));
}

Answer 2

要么你的模型（Meal、Beaverage 和 Dessert）也是 ViewModel，要么你的 DinnerViewModel 不是真正的 ViewModel。

ViewModel 的目的是为 View 提供直接可用的值。

根据您的单元测试示例，将您的 Calculator.Total 辅助方法直接放在 DinnerViewModel 并将总计算值作为属性公开给视图会更合适：

public class DinerViewModel
{
    public Meal Meal { get; set; }
    public Beverage Beverage { get; set; }
    public Desert Desert { get; set; }

    public decimal SalesTax { get; set; }
    public bool SeniorDiscount { get; set; }

    public decimal TotalCostOfDinner
    {
        get
        {
            decimal subtotal = 0M;
            if(Meal != null) subtotal += Meal.Cost;
            if(Beverage != null) subtotal += Beverage.Cost;
            if(Desert != null) subtotal += Desert.Cost;

            if (SeniorDiscount) subtotal -= subtotal * 0.1M;

            return subtotal + (subtotal * SalesTax);
        }
    }
}

现在您的视图可以直接从 DinnerViewModel 获取正确的 TotalCostOfDinner。

现在进行单元测试：

[TestMethod]
public void TotalCostOfDinnerReturnsCorrectTotalForNonSenior()
{
    DinnerViewModel dvm = new DinnerViewModel
    {
        Meal = new Meal { Cost = 7M },
        Beverage = new Beverage { Cost = 1M },
        Desert = new Desert { Cost = 2M },
        SalesTax = 0.08M,
        SeniorDiscount = false
    };


    decimal expected = 10.80M;
    decimal actual = dvm.TotalCostOfDinner;

    Assert.AreEqual(expected, actual, "The actual value does not match the expected value.");
}