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Basic Mocking / Asserting Occurrence
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Asserting Occurrence

Updated on Jan 27, 2026

Occurrence is used in conjunction with Mock.Assert and Mock.AssertSet to determine how many times a call has occurred.

There are 6 types of occurrence:

Furthermore, you can set occurrence directly in the arrangement of a method.

You can use one of 5 different constructs of Occur:

JustMock also enables you to verify the calls order, with:

  • InOrder() - Specifies exactly the order, a call should occur on the mock.

In addition to that JustMock allows you to verify that method call prerequisites are satisfied

In the following examples we will use the following interface to test:

C#
public interface IFoo
{
    void Submit();
    int Echo(int intArg);
}

Occurs.Never

Occurs.Never is used to verify that a method/property hasn't been called during the execution of the test.

C#
[TestMethod]
public void ShouldOccursNever()
{
    //Arrange
    var foo = Mock.Create<IFoo>();

    //Assert
    Mock.Assert(() => foo.Submit(), Occurs.Never());
}

In the example above, we verify that foo.Submit() has never been called.

Occurs.Once

Occurs.Once is used to verify that a method/property has been called exactly one time during the execution of the test.

C#
[TestMethod]
public void ShouldOccursOnce()
{
    //Arrange
    var foo = Mock.Create<IFoo>();

    //Act
    foo.Submit();

    //Assert
    Mock.Assert(() => foo.Submit(), Occurs.Once());
}

In the example above, we verify that foo.Submit() has been called once.

Occurs.AtLeastOnce

Occurs.AtLeastOnce is used to verify that a method/property has been called at least once during the execution of the test.

C#
[TestMethod]
public void ShouldOccursAtLeastOnce()
{
    //Arrange
    var foo = Mock.Create<IFoo>();

    //Act
    foo.Submit();

    //Assert 
    Mock.Assert(() => foo.Submit(), Occurs.AtLeastOnce());

    // Act - call Submit more than once
    foo.Submit();
    foo.Submit();

    // Assert - that should pass again
    Mock.Assert(() => foo.Submit(), Occurs.AtLeastOnce());
}

The example above shows that the Occurs.AtLeastOnce requirement is met whenever the method is called one or more times.

Occurs.AtLeast(numberOfTimes)

With Occurs.AtLeast you specify at least how many times you want to verify that a given method/property has been called during the execution of the test.

The next example uses NUnit Testing Framework.

C#
[TestMethod]
public void ShouldOccursAtLeastCertainNumberOfTimes()
{
    //Arrange
    var foo = Mock.Create<IFoo>();

    //Act
    foo.Submit();
    foo.Submit();

    //Assert
    NUnit.Framework.Assert.Throws<AssertFailedException>(() => Mock.Assert(() => foo.Submit(), Occurs.AtLeast(3)));

    foo.Submit();

    Mock.Assert(() => foo.Submit(), Occurs.AtLeast(3));
}

In the example above, the first Assert throws an exception as the foo.Submit method has been called only twice at that time, but the Occurs.AtLeast requires the method to be called at least 3 times. Afterwards, the method is called once again which causes the conditions in the second Assert to be satisfied.

Occurs.AtMost(numberOfTimes)

With Occurs.AtMost you specify at most how many times you want to verify that a given method/property has been called during the execution of the test.

C#
[TestMethod]
[ExpectedException(typeof(AssertFailedException))]
public void ShouldOccursCertainNumberOfTimesAtMost()
{
    //Arrange
    var foo = Mock.Create<IFoo>();

    //Act
    foo.Submit();
    foo.Submit();

    //Assert
    Mock.Assert(() => foo.Submit(), Occurs.AtMost(2));

    // Act - now we call Submit once again - 3 times in total
    foo.Submit();

    // Assert - that throws an exception
    Mock.Assert(()=>foo.Submit(),Occurs.AtMost(2));
}

In the example above, the first Assert passes as the foo.Submit method has been called only twice. However, the second Assert throws an exception because the same method has been called 3 times in total, while the condition is to be called 2 times at most.

Occurs.Exactly(numberOfTimes)

With Occurs.Exactly you specify exactly how many times you want to verify that a given method/property has been called during the execution of the test.

The next example uses NUnit Testing Framework.

C#
[TestMethod]
public void ShouldOccursExactly()
{
  //Arrange
  var foo = Mock.Create<IFoo>();

  //Act - call foo.Submit twice
  foo.Submit();
  foo.Submit();

  //Assert - throws an exception - foo.Submit was called only twice
  NUnit.Framework.Assert.Throws<AssertFailedException>(() => Mock.Assert(() => foo.Submit(), Occurs.Exactly(3)));

      // Act - call foo.Submit once again - 3 times in total
  foo.Submit();

  // Assert - that should pass
  Mock.Assert(() => foo.Submit(), Occurs.Exactly(3));

  // Act - call foo.Submit once again - 4 times in total
  foo.Submit();

  // Assert - fails because foo.Submit was called more times than specified
  NUnit.Framework.Assert.Throws<AssertFailedException>(() => Mock.Assert(() => foo.Submit(), Occurs.Exactly(3)));
}

In the example above, the first Assert throws an exception because we call foo.Submit less times than the Occurs.Exactly condition specifies. The second Assert passes because there are exactly 3 calls to foo.Submit method. The third Assert throws an exception again, because at than time the foo.Submit method is called 4 times, which is more that what is specified in the Occurs.Exactly condition.

Occurs(numberOfTimes)

With Occurs(numberOfTimes) you specify exactly how many times you want to verify that a given method/property has been called during the execution of the test.

The next example uses NUnit Testing Framework.

C#
[TestMethod]
public void ShouldFailOnAssertAllWhenExpectionIsNotMet()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Submit()).Occurs(2);

  // Assert
  NUnit.Framework.Assert.Throws<AssertFailedException>(() => Mock.Assert(foo));
}

In the example above, Occurs(2) marks the call is required to be executed 2 times, therefore Mock.Assert will fail if not expected properly.

OccursOnce()

OccursOnce() is used to verify that a method/property has been called exactly one time during the execution of the test.

C#
[TestMethod]
public void ShouldArrangeOccursOnce()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Submit()).OccursOnce();

  // Act
  foo.Submit();

  // Assert
  Mock.Assert(foo);
}

In the example above, we arrange that foo.Submit() must be called exactly once.

OccursNever()

OccursNever() is used to verify that a method/property hasn't been called during the execution of the test.

C#
[TestMethod]
public void ShouldArrangeOccursNever()
{
  // Arrange
  var foo = Mock.Create<IFoo>();
      
  Mock.Arrange(() => foo.Submit()).OccursNever();

  // Assert
  Mock.Assert(foo);
}

In the example above, we verify that foo.Submit() has never been called.

OccursAtLeast(numberOfTimes)

With OccursAtLeast(numberOfTimes) you specify at least how many times you want to verify that a given method/property has been called during the execution of the test.

C#
[TestMethod]
public void ShouldArrangeOccursAtLeast()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Submit()).OccursAtLeast(2);

  // Act
  foo.Submit();
  foo.Submit();
  foo.Submit();

  // Assert
  Mock.Assert(foo);
}

In the example above, foo.Submit() is called 3 times. The verification will not fail as it is expected to execute at least 2 times.

OccursAtMost(numberOfTimes)

With OccursAtMost(numberOfTimes) you specify at most how many times you want to verify that a given method/property has been called during the execution of the test.

C#
[TestMethod]
[ExpectedException(typeof(AssertFailedException))]
public void ShouldFailWhenInvokedMoreThanRequried()
{
  // Arrange
  var foo = Mock.Create<IFoo>();
      
  Mock.Arrange(() => foo.Submit()).OccursAtMost(2);

  // Act
  foo.Submit();
  foo.Submit();
  foo.Submit();
}

In the example above, we specify that foo.Submit() must be called at most 2 times. As it is called 3 times, AssertFailedException will be thrown immediately after the third call.

Asserting Multiple Occurrences

JustMock enables you to assert multiple occurrences of similar type of call using Matcher in Mock.Assert rather than asserting each of them separately.

Here is an example:

C#
[TestMethod]
public void ShouldBeAbleToAssertOccursUsingMatcherForSimilarCallAtOneShot()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Echo(1)).Returns((int arg) => arg);
  Mock.Arrange(() => foo.Echo(2)).Returns((int arg) => arg);
  Mock.Arrange(() => foo.Echo(3)).Returns((int arg) => arg);

  // Act
  foo.Echo(1);
  foo.Echo(2);
  foo.Echo(3);

  // Assert
  Mock.Assert(() => foo.Echo(Arg.AnyInt), Occurs.Exactly(3));
}

Our arrangement is for calling Echo method with 1, 2 or 3 as argument. In assertion we use a matcher to cover all calls with integer as argument. As calling foo.Echo with 1, 2 and 3 in acting phase applies that matcher, a call with integer occurred exactly 3 times. Refer to Matchers for more information about using matchers.

Verifying Calls Order

To ensure that a set of method calls are executed in a particular order, you can use the InOrder() method. To achieve this, you will need to use the following workflow:

  1. You arrange the methods to define their invocation order.
  2. You act.
  3. You assert on the mock, to check if the method execution order is as expected.

Next is an example, showing InOrder() use case:

C#
[TestMethod]
public void ShouldVerifyCallsOrder()
{
  // Arrange
  var foo = Mock.Create<IFoo>();

  Mock.Arrange(() => foo.Submit()).InOrder();
  Mock.Arrange(() => foo.Echo(Arg.AnyInt)).InOrder();

  // Act
  foo.Submit();
  foo.Echo(5);

  // Assert
  Mock.Assert(foo);
}

Note that the InOrder option also supports asserting the order of mock calls regardless of the instance within the test scope. Imagine that you have to validate that the user has logged in before using his/hers shopping cart in your application.

C#
public interface IUserValidationService
{
  int ValidateUser(string userName, string password);
}

public interface IShoppingCartService
{
  IList<string> LoadCart(int userID);
}

Here we have defined the IUserValidationService and the IShoppingCartService services whose invocation order we are going to assert in the following test:

C#
[TestMethod]
public void ShouldAssertInOrderForDifferentInstancesInTestMethodScope()
{
  string userName = "Bob";
  string password = "Password";
  int userID = 5;
  var cart = new List<string> { "Foo", "Bar" };

  // Arrange
  var userServiceMock = Mock.Create<IUserValidationService>();
  var shoppingCartServiceMock = Mock.Create<IShoppingCartService>();

  Mock.Arrange(() => userServiceMock.ValidateUser(userName, password)).Returns(userID).InOrder().OccursOnce();
  Mock.Arrange(() => shoppingCartServiceMock.LoadCart(userID)).Returns(cart).InOrder().OccursOnce();

  // Act
  userServiceMock.ValidateUser(userName, password);
  shoppingCartServiceMock.LoadCart(userID);

  // Assert
  Mock.Assert(userServiceMock);
  Mock.Assert(shoppingCartServiceMock);
}

In the arrange phase we have defined that the ValidateUser call should be made only once and before the LoadCart service call. The LoadCart call should also occur only once and should follow the ValidateUser service call. We act and then assert our expectations.

Verifying Call Prerequisites

JustMock gives you an ability to ensure that a method can be called only after one or more prerequisites have been called in prior. To achieve this use case, the following workflow is applicable:

  1. Arrange the methods to define the prerequisites.
  2. Specify the list all required prerequisites in AfterAll.
  3. Assert the target mock, to verify that all expected prerequisites have been called.

Next is an typical example, showing AfterAll() in action. Imagine that you have the following interfaces:

C#
public interface IBar
{
  void Init();
}

public interface IBarContainer
{
  IEnumerable<IBar> Elements { get; }
}
VB
Public Interface IBar
    Sub Init()
End Interface

Public Interface IBarContainer
    ReadOnly Property Elements As IEnumerable(Of IBar)
End Interface

and you need to verify that FooContainer.Elements is called after all Foo.Init have beeen called, than the following test can help:

C#
[TestMethod]
public void ShouldAssertPrerequisites()
{
  // Arrange
  var barCollection = new List<IBar>()
  {
      Mock.Create<IBar>(),
      Mock.Create<IBar>(),
      Mock.Create<IBar>()
  };
  var barContainer = Mock.Create<IBarContainer>();
  Mock.Arrange(() => barContainer.Elements)
          .ReturnsCollection(barCollection)
          .AfterAll(barCollection.Select(bar => Mock.Arrange(() => bar.Init())).ToArray());

  // Act
  barCollection.ForEach(bar => bar.Init());
  var actualCollection = barContainer.Elements;

  // Assert
  Mock.Assert(barContainer);
}
In this article
Occurs.NeverOccurs.OnceOccurs.AtLeastOnceOccurs.AtLeast(numberOfTimes)Occurs.AtMost(numberOfTimes)Occurs.Exactly(numberOfTimes)Occurs(numberOfTimes)OccursOnce()OccursNever()OccursAtLeast(numberOfTimes)OccursAtMost(numberOfTimes)Asserting Multiple OccurrencesVerifying Calls OrderVerifying Call Prerequisites
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