Invalid return type when testing Async methods

[Vikas]

I have 2 classes Parent and a Child class, child class inherits from parent.

Class under test has an async method that returns Task<parent>.

I am arranging the Test method in my unit test, Mock.Arrange(() => viewmodel.Test()).Returns(Task.FromResult(new child()));

I am getting this error, Why is it having a problem in casting a child to its parenttype

ErrorCS1503Argument 1: cannot convert from 'System.Threading.Tasks.Task<ViewModels.child>' to 'System.Threading.Tasks.Task<ViewModels.parent>'ViewModels.Tests

public class viewmodel{

public async Task<parent> Test()
        {
            await Task.Delay(100);
            return null;              
        }
    }

    public class parent
    {

    }

    public class child : parent
    {

    }

[Vikas]

If i change the arrange to this then it does not give compile time error:

Mock.Arrange(() => viewmodel.Test()).Returns(Task.FromResult<parent>(new child()));

But then i get invalid cast exception in the the code under test.

this line of code in the class under test throws can not cast from Threading.Task.Task<parent> to Threading.Task.Task<child>

child a  = await viewModel.Test() as child;

 

 

[Vikas]

It is working if i remove any expectations from the arrange like occurs(1). Does just mock supports expectations for async methods?

[Stefan]

Hi Vikas,

Task<child> is not convertible to Task<parent>, because in .NET only interfaces and delegates, but not classes, may have convariant generic parameters. Explicitly specifying the type parameter of FromResult<T> is the correct way.

This compiles:

Task<parent> task = Task.FromResult<parent>(new child());

This does not:

Task<parent> task = Task.FromResult(new child());

I tried the following code and I didn't get a type cast exception:

public class parent
{
}

public class child : parent
{
}

public interface ITest
{
    Task<parent> Test();
}

[TestMethod]
public async Task Test()
{
    var mock = Mock.Create<ITest>();
    Mock.Arrange(() => mock.Test())
        .Returns(Task.FromResult<parent>(new child()))
        .OccursOnce();

    child o = await mock.Test() as child;
}

A more complete example would help examine the issue.

Regards,
Stefan
Telerik

[Vikas]

This works, Thanks