XUnit/FsUnit框架抛出“对象引用未设置为objct实例”

Question

我为我的代码编写了这个简单的测试用例。

module CustomerTests
open Xunit
open FsUnit
open MyProject.Customer
open MyProject.Customer.Domain
    
module ``When upgrading customer`` = 
  let customerVIP = {Id = 1; IsVip = true; Credit = 0.0M}
  let customerSTD = {Id = 2; IsVip = false; Credit = 100.0M}
  [<Fact>]
  let ``should give VIP customer more credit`` () =
    let expected = {customerVIP with Credit = customerVIP.Credit + 100.0M }
    let actual = upgradeCustomer customerVIP
    actual |> should equal expected

非常令人惊讶的是，这段代码出错了。

[xUnit.net 00:00:00.64] CustomerTests+When upgrading customer.should give VIP cstomer more credit [FAIL]
  Failed CustomerTests+When upgrading customer.should give VIP cstomer more credit [3 ms]
  Error Message:
  System.NullReferenceException : Object reference not set to an instance of an object.
  Stack Trace:
    at CustomerTests.When upgrading customer.should give VIP cstomer more credit() in /Users/user/code/fsharp/CustomerProject/CustomerTests.fs:line 12

但是第12行只是一个正在创建的记录，因此该行不可能抛出对象引用，而不是设置为对象实例。这完全让我费解。

在dotnet fsi repl中，我可以执行我的所有方法，并且在我的函数中没有对象引用问题，这是从这里的测试中调用的。

Answer 1

正如this SO answer解释的那样，XUnit以跳过这些值初始化的方式加载测试。一个简单的解决方法是使用一个类而不是一个模块：

type ``When upgrading customer``() =

    let customerVIP = {Id = 1; isVip = true; Credit = 0.0M}
    let customerSTD = {Id = 2; isVip = false; Credit = 100.0M}

    [<Fact>]
    let ``should give VIP cstomer more credit`` () =
        let expected = {customerVIP with Credit = customerVIP.Credit + 100.0M }
        let actual = upgradeCustomer customerVIP
        actual |> should equal expected

这样，这些值的初始化就会按应有的方式进行。