Comparing Two Objects Using Assert.AreEqual()

Anyone who has ever Googled (Binged?) about unit testing have heard about the “one assert per test rule”. The idea is that every unit test should have only one reason to fail. It’s a good rule that helps me write good, robust unit tests — but like all such rules of thumb it’s not always right (just most of the time).

If you’ve been using unit tests for some time you might have come to a conclusion that using multiple asserts is not always a bad idea — in fact for some tests it’s the only way to go…

Consider the following class:

public class SomeClass
{
    public int MyInt { get; set; }
    public string MyString { get; set; }
}

And now imagine a test in which that SomeClass is the result of your unit tests – what assert would you write?

[TestMethod]
public void CompareTwoAsserts()
{
    var actual = new SomeClass { MyInt = 1, MyString = "str-1" };

    Assert.AreEqual(1, actual.MyInt);
    Assert.AreEqual("str-1", actual.MyString);
}

Using two asserts would work, at least for a time. The problem is that failing the first assert would cause an exception to be thrown leaving us with no idea if the second would have passed or failed.

We can solve this issue by splitting the test into two tests — one test per assert. This seems like overkill in this case - we’re not asserting for two different, unrelated “things”. We’re in fact testing one SomeClass that happen to have two properties.

Ideally I would have liked to write the following test: 

[TestMethod]
public void CompareTwoObjects()
{
    var actual = new SomeClass {MyInt = 1,MyString = "str-1"};
    var expected = new SomeClass {MyInt = 1,MyString = "str-1"};

    Assert.AreEqual(expected, actual);
}

Unfortunately it would fail. The reason is that deep down inside our assert have no idea what is an “equal” object and so it runs Object.Equals and throws an exception in case of failure. Since the default behavior of Equals is to compare references (in case of classes) the result is a fail.

Due to this behavior there are many (myself included) who suggest overriding Equals to make sure that the actual values are compared, which could be a problem if our production code cannot be changed to accommodate our tests. There are ways around this limitation, such as using a Helper class (ahem) that would do the heavy lifting by inheriting (or not) the original class and adding custom Equals code.

I propose another option – one that could be useful , especially when there’s a need to compare different properties in different tests.

Using Fake Objects to Compare Real Objects

In order to change the way two objects are compared in an assert we only need change the behavior of one of them – the expect value (might change depending on the unit testing framework). And who is better in changing behavior of objects in tests than your friendly-neighborhood mocking framework?

And so using FakeItEasy I was able to create the following code:

[TestMethod]
public void CompareOnePropertyInTwoObjects()
{
    var actual = new SomeClass { MyInt = 1, MyString = "str-1" };
    var expected = new SomeClass { MyInt = 1, MyString = "str-1" };

    var fakeExpected = A.Fake<someclass>(o => o.Wrapping(expected));

    A.CallTo(() => fakeExpected.Equals(A<object>._)).ReturnsLazily(
        call =>
        {
            var other = call.GetArgument<someclass>(0);

            return expected.MyInt == other.MyInt;
        });

    Assert.AreEqual(fakeExpected, actual);
}

What we have here is a new fake object a.k.a fakeExpected which would call custom code when its Equals method is called.

The new Equals would return true if MyInt is the same in the two objects. I’ve also created the new fake using Wrapping so that the original methods on the class would still be called – I really care about ToString which I would override to produce a meaningful assertion message.

Now all I needed to do is to compare the fakeExpected with the actual result from the test.

In a similar way I’ve created a new extension method that would compare the properties on two classes:

public static T ByProperties<T>(this T expected)
{
    var fakeExpected = A.Fake<T>(o => o.Wrapping(expected));

    var properties = expected.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public);

    A.CallTo(() => fakeExpected.Equals(A<object>._)).ReturnsLazily(
        call =>
        {
            var actual = call.GetArgument<object>(0);

            if (ReferenceEquals(null, actual))
                return false;
            if (ReferenceEquals(expected, actual))
                return true;
            if (actual.GetType() != expected.GetType())
                return false;

            return AreEqualByProperties(expected, actual, properties);
        });

    return fakeExpected;
}

private static bool AreEqualByProperties(object expected, object actual, PropertyInfo[] properties)
{
    foreach (var propertyInfo in properties)
    {
        var expectedValue = propertyInfo.GetValue(expected);
        var actualValue = propertyInfo.GetValue(actual);

        if (expectedValue == null || actualValue == null)
        {
            if (expectedValue != null || actualValue != null)
            {
                return false;
            }
        }
        else if (typeof (System.Collections.IList).IsAssignableFrom(propertyInfo.PropertyType))
        {
            if (!AssertListsEquals((IList) expectedValue, (IList) actualValue))
            {
                return false;
            }   
        }
        else if (!expectedValue.Equals(actualValue))
        {
            return false;
        }
    }

    return true;
}

private static bool AssertListsEquals(IList expectedValue, IList actualValue)
{
    if (expectedValue.Count != actualValue.Count)
    {
        return false;
    }

    for (int I = 0; I < expectedValue.Count; I++)
    {
        if (!Equals(expectedValue[I], actualValue[I]))
        {
            return false;
        }
    }

    return true;
}

And now I can use the following to compare my expected value with the value returned by the test:

[TestMethod]
public void CompareTwoObjectsByProperties()
{
    var actual = new SomeClass { MyInt = 1, MyString = "str-1" };
    var expected = new SomeClass { MyInt = 1, MyString = "str-1" };

    Assert.AreEqual(expected.ByProperties(), actual);
}

Simple(ish) is it? I prefer this method since I no longer need to make changes to my production code (e.g. SomeClass) but I can still use a plain vanilla unit testing framework.

What do you think?