单元测试中的依赖服务空引用异常

Question

I am trying to include Unit testing to my Xamarin PCL project.

This is what I am basically doing :

Login Page class :

 public bool isUserRemembered()
    {
        return DependencyService.Get<IUserDefaults>().getUserRemembered();
    }

    public void setRememberUser(bool check)
    {
        DependencyService.Get<IUserDefaults>().setUserRemembered(check);
    }

UnitTest class (iOS) :

    [Test]
    public void RememberKeyTrueTest()
    {
        LoginPage page = new LoginPage();
        bool isRemember = true;

        page.setRememberUser(isRemember);

        bool value = page.isUserRemembered();
        if (value)
            Pass();
        else
            Fail();
    }

I get Null Reference Exception on this line : DependencyService.Get<IUserDefaults>().setUserRemembered(check) .

Will dependency service work with unit testing? Or is there any work around it?

Answer 1

When I use Dependency Injection in my code, I use IoC (Inversion Of Control) to handle the creation of the dependencies in my production code, and manually create my dependencies in my unit tests.

My main reason for this is that it is explicit. Often I'll be using a Mocking framework to create fake instances so I don't want the extra complexity.

A suggestion on your code under test, you have swapped a dependency to your class for a dependency on your DependencyService. I'm not saying don't use the DependencyService but you could pass in the dependency to your constructor of the class. Then your test could pass in a fake instance and the production code would use the DependencyService to get you an IUserDefaults and pass it in. That would make your class easier to test.