编写基于Property-based的单元测试
使用FsCheck编写Property-based测试
在Xunit中使用FsCheck
使用FsCheck编写Model-based测试-待续
无论是Xunit还是Nunit都有额外的扩展用来编写FsCheck测试，以Xunit为例 ：
Install-Package FsCheck.Xunit -Version 2.13.0不同于普通的Xunit测试，一般的测试需要标记[Fact]，你需要使用[Property]标记FsCheck测试。给定一个函数:
private int Add(int x, int y) { return x + y; }针对加法交换律编写一个Property-based测试:
[Property] public bool Commutative(int x, int y) { return Add(x, y) == Add(y, x); }F#
[<Property>] let Commutative x y = add x y = add y x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:
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); }F#
[<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); }F#
[<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; }F#
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); } }F#
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 amountDollar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; }F#
[<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; }F#
[<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}; }