NSubstitute - mock throwing an exception in method returning Task

Question

Using NSubstitute, how do you mock an exception being thrown in a method returning a Task?

Let's say our method signature looks something like this:

Task<List<object>> GetAllAsync();

Here's how NSubstitute docs say to mock throwing exceptions for non-void return types. But this doesn't compile :(

myService.GetAllAsync().Returns(x => { throw new Exception(); });

So how do you accomplish this?

Answer 1

Actually, the accepted answer mocks a synchronous exception being thrown, that is not the real async behavior. The correct way to mock is:

var myService = Substitute.For<IMyService>();
myService.GetAllAsync()
         .Returns(Task.FromException<List<object>>(new Exception("some error")));

Let's say you had this code and GetAllAsync()

try
{
    var result = myService.GetAllAsync().Result;
    return result;
}
catch (AggregateException ex)
{
    // whatever, do something here
}

The catch would only be executed with Returns(Task.FromException>(), not with the accepted answer since it synchronously throws the exception.