Constructor Tests

Example 1 - Throws Exception

Test Case:

#	Test Case	Test Data	Expected
1	Exception when name is null	Parameter name: null, Parameter amountOfMoney: 500	ArgumentNullException, Parameter: name, Message: The argument must reference a string object.

Testing Goal: The goal of this test is to cause the expected exception to be thrown and the exception should have the expected state.

C#

[TestMethod]
public void Constructor_NameIsNull_ThrowsException()
{
    // Arrange
    string name = null;
    decimal amountOfMoney = 500;

    // Act & Assert
    ArgumentNullException exception = Assert.ThrowsException<ArgumentNullException>(() => new Person(name, amountOfMoney));

    // Assert exception state
    Assert.AreEqual("name", exception.ParamName);
    Assert.AreEqual("The argument must reference a string object.", GetExceptionMessage(exception.Message));
}

Notes:

• The act part of the test is new Person(name, amountOfMoney).
• The Assert.ThrowsException<T>() method generates a AssertFailedException if the type of exception is not thrown. If the type of exception is thrown, a reference to the exception object is returned.
• The assert part of the test includes verifying the parameter and message of the exception.

Non-Test Methods

Unit test class can also contain methods that aren't unit test methods. These methods do not have the TestMethod attribute.

The test above calls a method called GetExceptionMessage which extracts only the message from the Exception's Message property.

C#

/// <summary>
/// Helper method to obtain only the message from an Exception object.
/// </summary>
/// <param name="exceptionMessage">The Exception's Message state.</param>
/// <returns>The Exception's message with the parameter omitted.</returns>
/// <remarks>
/// The Exception.Message property returns the Exception's message on line 1 and
/// the parameter name on line 2. This method reads the first line and returns
/// the message.
/// </remarks>
private string GetExceptionMessage(string exceptionMessage)
{
    return new System.IO.StringReader(exceptionMessage).ReadLine();
}

Feel free to include this method in your test class.

Example 2 - Initialize State

Test Case:

#	Test Case	Test Data	Expected
6	Initialize the amount of money to positive value	Parameter name: "Kenny", Parameter amountOfMoney: 500	500

Testing Goal: The goal of this test is to initialize the state of the object and verify it did so correctly.

C#

[TestMethod]
public void Constructor_ValidAmountOfMoney_InitializeState()
{
    // Arrange
    string name = "Kenny";
    decimal amountOfMoney = 500;

    // Act
    Person person = new Person(name, amountOfMoney);

    // Reflection
    PrivateObject target = new PrivateObject(person);

    // Obtain object state
    decimal actual = (decimal)target.GetField("amountOfMoney");

    // Assert
    Assert.AreEqual(amountOfMoney, actual);
}

Notes:

• Because the state is stored in a private field, the PrivateObject class is used to gain access to the class' private member.
• The GetField() method returns an object type. Because fields can be any type, the method uses this polymorphic reference to the value of the field. A type cast is necessary to store the value in the decimal variable.

Testing in Isolation

One of the key parts to writing good unit tests is to test the unit in isolation. This means not directly invoking any other class members during the test. Testing in isolation ensures that no other part of the class is affecting the results of your test.

The following example, is a revised version of the test code from above. This example does not test the constructor method in isolation, as the get accessor or the Name property is invoked later in the test method.

C#

[TestMethod]
public void Constructor_ValidAmountOfMoney_InitializeState()
{
    // Arrange
    string name = "Kenny";
    decimal amountOfMoney = 500;

    // Act
    Person person = new Person(name, amountOfMoney);

    // Obtain object state
    decimal actual = person.AmountOfMoney;

    // Assert
    Assert.AreEqual(amountOfMoney, actual);
}

Notes:

• This test does not test the unit in isolation.