Moq: Setup a mocked method to fail on the first call, succeed on the second

Question

What's the most succinct way to use Moq to mock a method that will throw an exception the first time it is called, then succeed the second time it is called?

Moq 4.2 & higher now supports this via `SetupSequence()`. – Richard Barnett Feb 26 '15 at 00:41 — Richard Barnett, Feb 26 '15 at 00:41

rsbarro · Accepted Answer · 2011-08-12T20:14:48.950

69

I would make use of Callback and increment a counter to determine whether or not to throw an exception from Callback.

[Test]
public void TestMe()
{
    var count = 0;
    var mock = new Mock<IMyClass>();
    mock.Setup(a => a.MyMethod()).Callback(() =>
        {
            count++;
            if(count == 1)
                throw new ApplicationException();
        });
    Assert.Throws(typeof(ApplicationException), () => mock.Object.MyMethod());
    Assert.DoesNotThrow(() => mock.Object.MyMethod());
}

public interface IMyClass
{
    void MyMethod();
}

edited Aug 12 '11 at 20:14

answered Aug 12 '11 at 19:58

rsbarro

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this is a good answer and this approach becomes preferable once you want more than 2 things to happen. – anthony Aug 12 '11 at 20:50
This approach is simple which is nice, but check that post by Phil Haack from @Mathias' answer. It's pretty slick. Using an extension method you can do something like `reader.Setup(r => r.Read()).ReturnsInOrder(true, true, false);` – rsbarro Aug 12 '11 at 20:52
5

ReturnsInOrder() is nice if you want to return a sequence of values but it wasn't readily apparent how to use it to throw an exception as part of the sequence. – anthony Aug 12 '11 at 20:56
3

This answer is not wrong, but using `SetupSequence` is easier and more elegant for sure. Check @aghidini's answer. – Martijn Oct 07 '21 at 13:21

score 65 · Answer 2 · answered Aug 21 '17 at 10:14

65

Starting with Moq 4.2 you can just use the built-in method SetupSequence() (as stated by @RichardBarnett comment).

Example:

var mock = new Mock<IMyClass>();
mock.SetupSequence(x => x.MyMethod("param1"))
    .Throws<MyException>()
    .Returns("test return");

answered Aug 21 '17 at 10:14

aghidini

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5

This only works if `MyMethod` has a return type. If the method is `void` then you can't use this approach. Would be nice if there were a way to do this with `void` methods – Simon Jan 29 '20 at 10:58

score 18 · Answer 3 · answered Aug 12 '11 at 19:57

18

The best that I've come up with so far is this:

interface IFoo
{
    void Bar();
}

[Test]
public void TestBarExceptionThenSuccess()
{
    var repository = new MockRepository(MockBehavior.Default);
    var mock = repository.Create<IFoo>();

    mock.Setup(m => m.Bar()).
        Callback(() => mock.Setup(m => m.Bar())). // Setup() replaces the initial one
        Throws<Exception>();                      // throw an exception the first time

    ...
}

answered Aug 12 '11 at 19:57

anthony

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2

+1 That's a cool approach. Just might not be clear to someone stumbling into the code what's going on there. – rsbarro Aug 12 '11 at 20:00
Nice one, I didnt think of this! – Erin Drummond Aug 05 '16 at 09:49
5

Note that replacing the mock wipes out the history so you can't do `mock.Verify(m => m.Bar(), Times.Exactly(2))`. – giraffe.guru Dec 10 '17 at 21:10

score 7 · Answer 4 · answered Aug 12 '11 at 20:15

Phil Haack has an interesting blog post on setting up a method to return a particular sequence of results. It seems that it would be a good starting point, with some work involved, because instead of a sequence of values of a certain type, you would need now to have a sequence of results which could be of type T, or an exception.

Moq: Setup a mocked method to fail on the first call, succeed on the second

4 Answers4