如何对ViewModel使用方法进行单元测试？

Question

我想测试一个将ViewModel作为参数的帮助器方法。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。在我给出的例子中，这不是什么大不了的，因为只有少数几个，但是在我工作的实际虚拟机上有一吨。有没有其他的方式来做到这一点，所以我不必创建和分配每个对象？出于说明的目的

实施例的代码...

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; } 
} 

的视图模型

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; } 
} 

辅助

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."); 
} 

这将导致一个 “NullReferenceException” 错误。正如我所说的，我可以创建并分配必要的对象...

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

...而且，一旦他们完成所有操作，测试成功，但是这似乎是一个更大的虚拟机大量的工作我觉得有一些事情我可以做，让这更容易。

Answer 1

您看到NullReferenceException，因为在实例化时，Meal，Beverage和Desert实例未分配给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：

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; } 
} 

而且我喜欢创造一些辅助方法，返回存根测试。它消除了重复，使你的测试明确：

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

无论你的模型（膳食，Beaverage和甜品）是的ViewModels过，或者你DinnerViewModel是不是一个真正的视图模型。

ViewModel的目的是为View提供直接可用的值。

根据您的单元测试例如，它会更合适，直接把你的Calculator.Total helper方法的DinnerViewModel和揭露计算出的总价值为属性的观点：

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); 
     } 
    } 
} 

现在您查看可以直接从DinnerViewModel中获取正确的TotalCostOfDinner。

现在的单元测试：

[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."); 
}