在Xunit中使用FsCheck

在之前的例子里，我们介绍了什么是Property-based测试，然后花了一篇博客介绍了各种各样的Generator，每一个刚开始了解Property-based测试的人都会觉得这种方案很有意思，但是当你真正开始编写Property-base测试的时候你就会感觉得无从下手，应该断言什么样的Properties呢？

这篇文章介绍一些Properties供你参考:

1. 不同的执行顺序，同样的执行结果

例如被测函数为List.OrderBy，如果我们在List.OrderBy函数之前执行一个操作Add1，然后执行List.OrderBy函数。结果应该等于先执行List.OrderBy函数再执行操作Add1

 [Property]

public bool AddOneThenSortShouldSameAsSortThenAddOne(List<int> list)

{

    var result1 = list.OrderBy(x => x).Select(Add1);

    var result2 = list.Select(Add1).OrderBy(x => x);

    return result1.SequenceEqual(result2);

}

 [<Property(Verbose=true)>]

let ``+1 then sort should be same as sort then +1`` aList =

    let add1 x = x + 1

    let result1 = aList |> List.sort |> List.map add1

    let result2 = aList |> List.map add1 |> List.sort

    result1 = result2

2.连续执行操作，结果跟之前一致

例如List.Reverse函数，连续执行两次，结果跟期初是一样的。类似的函数如序列化和反序列化，Redo和Undo。

[Property]

public bool ReverseThenReverseShouldSameAsOriginal(int[] list)

{

  var result=  list.Reverse().Reverse();

  return result.SequenceEqual(list);

}

[<Property>]

let ``reverse then reverse should be same as original``

(aList:int list) =

    let reverseThenReverse = aList |> List.rev |> List.rev

    reverseThenReverse = aList

3. 有一些属性是永远不会改变的

在数据变化过程中，有一些属性是永远不会改变的，例如Sort操作，前后数据的Length总是不变的，这一属性可以作为Property-based测试的一个依据:

public bool SomethingNeverChanged(List<int> list)

{

    var result = list.OrderBy(x => x);

    return result.Count() == list.Count;

}

let ``sort should have same length as original`` (aList:int list) =

    let sorted = aList |> List.sort

    List.length sorted = List.length aList

为OO代码编写Property-based测试

接下来我们尝试针对一个OO的例子编写Property-based测试:

public class Dollar

{

    private int _amount;

    public Dollar(int amount)

    {

        _amount = amount;

    } 

    public int Amount => _amount;

    public void Add(int add)

    {

         _amount = _amount + add;

    }

    public void Multiplier(int multiplier)

    {

        _amount = _amount * multiplier;

    }

    public static Dollar Create(int amount)

    {

        return new Dollar(amount);

    }

}

type Dollar(amount : int) =

    let mutable privateAmount = amount;

    member this.Amount = privateAmount

    member this.Add add =

        privateAmount <- this.Amount + add

    member this.Times multiplier =

        privateAmount <- this.Amount * multiplier

    static member Create amount =

        Dollar amount

Dollar类主要有两个方法，Add和Multiplier分别用来修改私有变量_amount。如何测试Dollar类呢？都有哪些Properties可用？调用Add方法后再读取Amount的值应该是同一个值：

[Property]

public bool SetAndGetShouldGiveSameResult(int amount)

{

    var dollar = Dollar.Create(0);

    dollar.Add(amount);

    return dollar.Amount == amount;

}

[<Property>]

let ``set then get should give same result`` value =

    let obj = Dollar.Create 0

    obj.Add value

    let newValue = obj.Amount

    value = newValue

还有什么Property可供使用呢，Add和Multiplier两个方法执行完毕的结果等价于直接Create:

[Property]

public bool AddThenMultiplierSameAsCreate(int start, int times)

{

    var dollar = Dollar.Create(0);

    dollar.Add(start);

    dollar.Multiplier(times);

    var dollar2 = Dollar.Create(start * times);

    return dollar.Amount == dollar2.Amount;

}

 [<Property>]

let ``add then multiplier same as create`` value times =

    let dollar = Dollar.Create 0

    dollar.Add value

    dollar.Times times

    let dollar2 = Dollar.Create(value*times);

    dollar.Amount = dollar2.Amount

编写自定义Generator

迄今为止，我们都在使用FsCheck自带的Generator，而在实际项目开发过程中，你还需要生成自定义的Generator供你使用，例如有一个User类型：

public class User

{

    public string Name { get; set; }

    public int Age { get; set; }

}

自定义Generator:

public class UserArbitrary: Arbitrary<User>

{

    public override Gen<User> Generator =>

        from x in Arb.Generate<string>()

        from int y in Gen.Choose(20, 30)

        where x != string.Empty

        select new User {Name = x, Age = y};

}

最后还要将自定义的Arbitrary注册在FsCheck中：

public class MyGenerators {

    public static Arbitrary<User> User() {

        return new UserArbitrary();

    }

}

Arb.Register<MyGenerators>();

写个例子试试：

[Property]

public bool GenerateUsers(User user)

{

   return user.Name != string.Empty;

}

所以代码实例均可以在github下载

巴特西