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.