为什么 Razor 页面处理程序返回类型的 PartialViewResult 的单元测试失败？

Question

I am using Core 3.1 Xunit to test a Razor Page in an application, with Moq to mock any services. When running a test on a handler of PageModel to test if it returns PartialViewResult type I get the following exception:

System.NullReferenceException : Object reference not set to an instance of an object.

The PageModel handler looks like this, with Application as a property on PageModel :

public PartialViewResult OnGetApplicationAddedModalPartial()
{
    return Partial("_ApplicationAddedModalPartial", Application);
}

And this is the failing test for it:

[Fact]
public void OnGetApplicationAddedModalPartial_WhenCalled_ShouldReturPartialViewResultType()
{
    //Arrange
    _sut.Application = It.IsAny<ApplicationModel>();

    //Act
    var result = _sut.OnGetApplicationAddedModalPartial();

    //Assert
    Assert.IsType<PartialViewResult>(result);
}

What's curious is that I have another handler in that PageModel that I successfully test to see if the return is of JasonResult type. Additionally, I have controllers in this application of which some actions also return a PartialViewResult type that I successfully test for.

What is special about a handler of return type PartialViewResult on a PageModel and how can I create a test to ensure that it returns PartialViewResult ?

Answer 1

When testing a PartialViewResult , you need to instantiate a number of properties of the PageModel as part of your setup: https://github.com/dotnet/AspNetCore.Docs/issues/24312

// Arrange
var httpContext = new DefaultHttpContext();
var modelState = new ModelStateDictionary();
var actionContext = new ActionContext(httpContext, new RouteData(), new PageActionDescriptor(), modelState);
var modelMetadataProvider = new EmptyModelMetadataProvider();
var viewData = new ViewDataDictionary(modelMetadataProvider, modelState);
var tempData = new TempDataDictionary(httpContext, Mock.Of<ITempDataProvider>());
var pageContext = new PageContext(actionContext)
{
    ViewData = viewData
};
var pageModel = new MyPageModel()
{
    PageContext = pageContext,
    TempData = tempData,
    Url = new UrlHelper(actionContext),
    MetadataProvider = modelMetadataProvider
};

// Act    
var result = await pageModel.GetPartial();

// Assert
Assert.IsType<PartialViewResult>(result);