How to handle delegate parameter in NSubstitute

Question

I have a class with a method that has a signature like this:

public async Task<ResponseType> getSuff(string id, 
                                        string moreInfo, 
                                        deletegateName doStuff
                                       ) { // details. }

I am trying to mock this call with NSubstitute like this:

MyClass helper = Substitute.ForPartsOf<MyClass>();
ResponseType response = new ResponseType();
helper.getSuff(Arg.Any<string>(), 
               Arg.Any<string>(),
               Arg.Any<DelegateDefn>()).Returns(Task.FromResult(response));

But I am getting a runtime error:

NSubstitute.Exceptions.CouldNotSetReturnDueToNoLastCallException: Could not find a call to return from.

Make sure you called Returns() after calling your substitute (for example: mySub.SomeMethod().Returns(value)), and that you are not configuring other substitutes within Returns() (for example, avoid this: mySub.SomeMethod().Returns(ConfigOtherSub())).

If you substituted for a class rather than an interface, check that the call to your substitute was on a virtual/abstract member. Return values cannot be configured for non-virtual/non-abstract members.

Correct use: mySub.SomeMethod().Returns(returnValue);

Potentially problematic use: mySub.SomeMethod().Returns(ConfigOtherSub()); Instead try: var returnValue = ConfigOtherSub(); mySub.SomeMethod().Returns(returnValue);

The problem is, I think, the delegate. I just want to mock out the method, I do not want the delegate actually passed to be executed.

So, does anyone know how I can achieve this? Thanks.

Answer 1

NSub works perfectly with delegates and async code. Here's a simple example:

public delegate void SomeDelegate();
public class Foo
{
    public virtual async Task<int> MethodWithDelegate(string something, SomeDelegate d)
    {
        return await Task.FromResult(1);
    }
}

[Test]
public void DelegateInArgument()
{
    var sub = Substitute.For<Foo>();
    sub.MethodWithDelegate("1", Arg.Any<SomeDelegate>()).Returns(1);
    Assert.AreEqual(1, sub.MethodWithDelegate("1", () => {}).Result);
    Assert.AreNotEqual(1, sub.MethodWithDelegate("2", () => {}).Result);
}

Please make sure that the method you specify is virtual or abstract. From the exception you got:

If you substituted for a class rather than an interface, check that the call to your substitute was on a virtual/abstract member. Return values cannot be configured for non-virtual/non-abstract members.