NSubstitute ambiguous call when following documentation example (but with async method)

Question

So I tried copying an example about exception throwing from the documentation, and added this to one of my methods:

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

But this results in the following compiler error:

Error CS0121 The call is ambiguous between the following methods or properties: 'SubstituteExtensions.Returns(T, Func, params Func[])' and 'SubstituteExtensions.Returns(Task, Func, params Func[])'

The method I'm using is an async method. I tried awaiting the method first, but that returns a different error, saying that it

can not return value of type X for (expected type Task`1)

UPDATE: I can change the lambda in the returns to be async as a way to get around the compiler error, but this results in a warning. So I guess the question now is more about whether or not there's a way to bypass this without having the warning show up, and without adding needless await code inside the lambda itself?

Is there a way to avoid this? I'm using NSubstitute 2.0.3

Thank you for any help.

score 14 · Accepted Answer · answered Jun 27 '17 at 23:55

The compiler error in your question looks like you are calling something that returns Task rather than Task<T>? This sample works for me:

public interface ISample {
    Task DoStuff(string name);
}

[Test]
public async Task SampleTest()
{
    var sample = Substitute.For<ISample>();
    sample.DoStuff("test").Returns(x => { throw new Exception("doh"); });
    // ...
}

I tend to get that error when I have a generic Task<T>, which can be fixed by specifying the return type in the .Returns() call like this:

public interface IRepository {
    Task<int> FindId(string name);
}

[Test]
public async Task SampleTest2()
{
    var sample = Substitute.For<IRepository>();
    sample.FindId("test").Returns<int>(x => { throw new Exception("doh"); });
    // ...
}

In the example above I've removed the ambiguity by using .Returns<int>(...) which will pick the first overload mentioned in the compiler error.

If that does not help could you post the signature of the method being tested?

Can you add the information to NSubstitute documentation? I've hit the same problem second time. Or consider some name changes to avoid ambiguity? — Michael Freidgeim, Sep 24 '19 at 06:36
I fell over this issue too. I think updating the NSubstitute documentation to help people trying to throw errors when returning from `Task<>` would be a tremendous help. Something like the answer from @m12lrpv would be fabulous. — Zodman, Mar 09 '21 at 08:39

score 5 · Answer 2 · answered Aug 05 '19 at 23:11

Adding to Davids accepted answer if you want the Task method because your method does actually return a Task then you simply need to specify Returns<Task>.

object.Method().Returns<Task>(x => { throw new Exception("You messed up"); });

It's not necessary to specify the full return type i.e. don't do this -> Task<IEnumerable<YourClass>>

NSubstitute ambiguous call when following documentation example (but with async method)

2 Answers2