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Assertion uses exactly the same syntax as configuration to specify the call to be asserted, followed by a method call beginning with .MustHaveHappened.

The two most common forms of assertion are :

  • MustHaveHappened() (no arguments) asserts that the call was made 1 or more times, and
  • MustNotHaveHappened() asserts that the specified call did not happen at all.

Arguments are constrained using Argument Constraints just like when configuring calls.


A.CallTo(() => foo.Bar()).MustHaveHappened();
A.CallTo(() => foo.Bar()).MustNotHaveHappened();

A.CallTo(() => foo.Bar()).MustHaveHappenedOnceExactly();
A.CallTo(() => foo.Bar()).MustHaveHappenedOnceOrMore();
A.CallTo(() => foo.Bar()).MustHaveHappenedOnceOrLess();

A.CallTo(() => foo.Bar()).MustHaveHappenedTwiceExactly();
A.CallTo(() => foo.Bar()).MustHaveHappenedTwiceOrMore();
A.CallTo(() => foo.Bar()).MustHaveHappenedTwiceOrLess();

A.CallTo(() => foo.Bar()).MustHaveHappened(4, Times.Exactly);
A.CallTo(() => foo.Bar()).MustHaveHappened(6, Times.OrMore);
A.CallTo(() => foo.Bar()).MustHaveHappened(7, Times.OrLess);

A.CallTo(() => foo.Bar()).MustHaveHappenedANumberOfTimesMatching(n => n % 2 == 0);

Asserting Calls Made with Mutable Arguments

When FakeItEasy records a method (or property) call, it remembers which objects were used as argument, but does not take a snapshot of the objects' state. This means that if an object is changed after being used as an argument, but before argument constraints are checked, expected matches may not happen. For example,

var aList = new List<int> {1, 2, 3};

A.CallTo(() => myFake.SaveList(A<List<int>>._))


A.CallTo(() => myFake.SaveList(A<List<int>>.That.IsThisSequence(1, 2, 3)))

The MustHaveHappened will fail, because at the time the IsThisSequence check is made, aList has 4 elements, not 3, and IsThisSequence only has the reference to aList to use in its check, not a deep copy or some other form of snapshot—it has to work with the current state.

If your test or production code must mutate call arguments between the time of the call and the assertion time, you must look for some other way to verify the call. Perhaps using IsSameAs will suffice, if the correct behavior of the System Under Test can otherwise be inferred. Or consider using Invokes to create a snapshot of the object and interrogate it later:

var aList = new List<int> {1, 2, 3};

List<int> capturedList;
A.CallTo(() => myFake.SaveList(A<List<int>>._))
    .Invokes((List<int> list) => capturedList = new List<int>(list))


Assert.That(capturedList, Is.EqualTo(new List<int> {1, 2, 3}));

More advanced assertions

If the built-in assertion API isn't sufficient, you can also examine the list of recorded calls directly, as described in Getting the list of calls made on a fake.


' Functions and Subs can be asserted using their respective keywords
A.CallTo(Function() foo.Bar()).MustHaveHappened()
A.CallTo(Sub() foo.Baz(A(Of String).Ignored)).MustHaveHappened()