How to mock a method with CancellationToken passed as parameter?

Question

I'm recently learning and using Polly to add resilience to my code, especially for the timeout and retry policies. However, I don't know how to unit test the code with polly. To be more specific, I don't know how to mock a method with Cancellation Token as its parameter. Below is the structure of my code

public class Caller
{
     private IHttpManager httpManager;
     private IAsyncPolicy<HttpResponseMessage> policyWrap;

     public Caller(IHttpManager httpManager, IAsyncPolicy<HttpResponseMessage> policyWrap)
     {
        this.httpManager= httpManager;
        this.policyWrap = policyWrap;
     }


     public async Task CallThirdParty()
     {      

        HttpResponseMessage httpResponse = await policyWrap.ExecuteAsync(async ct => await httpManager.TryCallThirdParty(ct), CancellationToken.None);

     }

}

public interface IHttpManager
{
    Task<HttpResponseMessage> TryCallThirdParty(CancellationToken cancellationToken);
}

---

Below is the unit test I intend to run but don't know how.

[Test]
public void TestBehaviourUnderTimeoutPolicy()
{
     // Set up the timeout policy such that the governed delegate will terminate after 1 sec if no response returned
     AsyncTimeoutPolicy<HttpResponseMessage> timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(1, TimeoutStrategy.Optimistic);


     // mock IHttpManager
     var mockHttpManager= new Mock<IHttpManager>();

     // THIS IS WHERE I'M HAVING TROUBLE WITH.
     // I want to simulate the behaviour of the method such that
     // it will throw an exception whenever the CancellationToken passed from the polly caller expires
     // But how can I do that with mock?
     mockHttpManager.Setup(m => m.TryCallThirdParty(It.IsAny<CancellationToken>()))).Returns(Task.Run(() => { Thread.Sleep(10000); return new HttpResponseMessage(); }));

     Caller c = new Caller(mockHttpManager.Object, timeoutPolicy);
     await c.CallThirdParty();

}

Answer 1

@Noremac is right (you test 2 elements in one unit test).

However, to answer the question "How to make Mock<T> throw an exception when CancellationToken is cancelled": Use Returns<TParam1, ...>(Func<TResult, TParam1>) overload. I replaced AsyncTimeoutPolicy with CancellationTokenSource for clarity.

// This test will perma-hang if the exception is not thrown
[TestMethod]
[ExpectedException(typeof(OperationCanceledException))]
public async Task TestMethod1()
{
    var source = new Mock<IHttpManager>();
    source.Setup(s => s.TryCallThirdParty(It.IsAny<CancellationToken>())).Returns<CancellationToken>(
        async token =>
        {
            // Wait until the token is cancelled
            await Task.Delay(Timeout.Infinite, token);
        });

    var tcs = new CancellationTokenSource(1000);
    await new Caller().Get(source.Object, tcs.Token);
}