Tuesday, 26 July 2011

Unit Testing C# Custom Attributes with NUnit Part 3

During Unit Testing C# Custom Attributes with NUnit Part 2 where the Assertions were converted from the Classic to the Constraint model it was simply a process of replacing



Assert.That(<object>, Is.<SomeAssertion>)

The key thing being the 'Is' object that houses all the original assertions used.  Whilst doing this I came across the 'Has' object.  This doesn't appear in the documentation until about halfway when collections are covered.  Not mentioned at all in the documentation is the method Has.Attribute<AttributeType> which tests whether an attribute (custom or otherwise) is present on the object being tested.   For the current method this is simply:

Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>());

Even better, having obtained the Attribute (if not present the assertion will fail) it too can be tested.  The important aspect here is that it contains a specific property which can easily be tested in the same statement by appending '.Property(<PropertyName>)' in a Fluent style to give:

Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("FunkyName"));

The next is to test whether this property contains the correct value.  This can be obtained in a similar way by further appending '.EqualTo(<SomeValue>)' to give:

Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("FunkyName").EqualTo("RipSnorter"));

In a single statement tests for the presence of the Custom Attribute, a property of it and finally that property's value have been conducted.  This is considerably shorter than both the original and second examples which required the reflection code to obtain the Custom Attribute followed by 3 separate assertions.

This does not meet the original testing requirements which were:

  • A Custom Attribute of the correct type existed on the test method.
  • That the property to be tested of the Custom Attribute had the expected name.
  • That the property to be tested of the Custom Attribute had the correct type.
  • That the value of property to be tested of the Custom Attribute could be set.
  • That the value of property to be tested of the Custom Attribute could be obtained.
  • That the property to be tested of the Custom Attribute had the expected value.

Remaining are setting, getting (obtain) and type checking.

It turns out that that the tests for being able to set and get the property are not needed as if a property is created on a Custom Attribute then a getter and setter must be supplied.  The property can be private but if this is the case then it cannot be set as part of the Attribute syntax.  If the former condition is not met or an attempt is made to set a private property then a compilation error will occur.

Whilst successfully testing the property's value would suggest a type match this it not strictly the case as if the expected value can be converted to the type of the property then the test will be successful, e.g.

Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("SettingOne").EqualTo(77.0));

which causes the double to be converted to an int.  It wouldn't be possible to actually set 'SettingOne' to a double value when using the attribute as this will fail to compile, e.g.

[Funky(SettingOne = 77.9)]

As this is testing an implementation of a Custom Attribute rather than its application then it is necessary to check that the implementation hasn't been accidentally changed to allow this in which case the previous erroneous test would pass.  This means testing the type.

Unfortunately this is where the Fluent interface of NUnit's Constraint model falls downs a little as it's not possible to perform multiple tests on the same initial subject which in this case is the MethodBase returned from the static MethodBase.GetCurrentMethod() call.  Therefore an additional assertion is required:

Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("SettingOne").TypeOf<int>());

This means the original example can be reduced to the following:

//[Funky(FunkyName = "RipSnorter")]
public void TestThatNameIsRipSnorter()
 Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("FunkyName").EqualTo("RipSnorter"));
 Assert.That(MethodBase.GetCurrentMethod(), Has.Attribute<FunkyAttribute>().Property("FunkyName").TypeOf<string>());

[Funky(SettingOne = 77)]
public void TestThatSettingOneIs77()
 // Two asserts

 // Use of '.And'.  Note the trailing '.'
// Use of overloaded '&'
           & Has.Attribute<FunkyAttribute>().Property("SettingOne").EqualTo(77));

with the Custom Attribute definition remaining as

public class FunkyAttribute : Attribute
 public int SettingOne { get; set; }
 public string FunkyName { get; set; }
In the TestSettingOneIs77 method the need for two separate assertions has been slightly improved upon.  This is by using the 'And' method which is of the Fluent style.  The final assertion is exactly the same as the previous but just demonstrates the overloaded '&' syntax instead.  

I don't think either of these styles is particularly better than the two line equivalent as they both require two calls to obtain the Custom Attribute; one for each test.  However, using NUnit's Constraint model coupled with Fluent interface reduces the required code dramatically so is worthwhile.  Additionally I'm not sure if the Classic model actually allows Attributes to be obtained.  

A test per-property on a Custom Attribute is probably also desirable but there's nothing stopping you combining all the individual property tests into a single assertion by the use of  '.And.HasAttribute().Property().EqualTo()'.  One of these would be required  for each of the remaining properties along with a similar one for the type check.  A test per-property is more readable.

I think I'm done for a while on this subject now!

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