使用Prism for Xamarin Forms时出现nUnit测试问题

Question

我正在尝试使用nUnit为Xamarin Forms项目编写单元测试。但是当我在await vm.LoadExes();中调用方法Viewmodel时，我收到了错误：

  

类型＆＃39; BindableBase＆＃39;在未引用的程序集中定义。您必须添加对程序集的引用＆＃39; Prism，Version = 6.2.0.0，Culture = neutral，PublicKeyToken = 91a96d2a154366d8＆＃39;。＆＃34;

    public class ExesTests 
    {
        private Mock<IExService> mockExService;
        private Mock<INavigationService> mockNavigationService;
        private Mock<IPageDialogService> mockPageDialogService;

        [TestInitialize]
        public void Setup()
        {
            mockExService = new Mock<IExService>();
            var exes = new List<Ex>()
            {   
                new Ex()
            };
            mockExService.Setup(o => o.GetExes().Result).Returns(exes);
        }

        [TestMethod]
        public async Task GetExesTest()
        {           
            var vm = new ExesPageViewModel(mockExService.Object, 
                mockNavigationService.Object, mockPageDialogService.Object);            
            await vm.LoadExes(); // Getting error here

         //Assert here
        }
    }

Prism用于MVVM的Xamarin Forms项目 ViewModel中方法的签名是

internal async Task LoadExes()
{
    //Do something
}

方法标记为internal，以便可以从单元测试项目访问。