default(T) behaviour on anonymous types

Question

I'm in the process of writing unit tests against our core framework, and came across this.

We have an extension method that looks like this:

    public static T ThrowIfDefault<T>(this T self, string variableName)
    {
        if (self.Equals(default(T)))
            throw new ArgumentException(string.Format("'{0}' cannot be default(T)", variableName));
        return self;
    }   // eo ThrowIfDefault<T>

(A variation on a ThrowIfNull<> extension method I saw here on Stack Overflow.

In writing a test case for this, I first wrote a helper:

    public void ThrowIfDefaultTestHelper<T>(T value)
    {
        // unit test *itself* requires that a value be specified!!
        Assert.AreNotEqual(default(T), value);

        // Good test
        GenericExtensionMethods.ThrowIfDefault(value, "value");

        // Bad test
        try
        {
            GenericExtensionMethods.ThrowIfDefault(default(T), "value");
        }
        catch (ArgumentException)
        {
            // Expected result
        }
        catch (Exception)
        {
            throw;
        }
    }

And then the following:

    [TestMethod()]
    public void ThrowIfDefaultTest()
    {
        ThrowIfDefaultTestHelper<int>(10);
        ThrowIfDefaultTestHelper<Guid>(Guid.NewGuid());
        ThrowIfDefaultTestHelper<DateTime>(DateTime.Now);
        ThrowIfDefaultTestHelper<object>(new { Name = "Test" });    // anonymous object
    }

The Unit test fails on the last one as a NullReferenceException is thrown, because I am guessing object has no default(T) (or does it?). Can I not test anonymous objects this way?

Answer 1

object does have default(T), it just happens to be null. This is unlike your other test cases that use non-nullable value types. That's why you get a NullReferenceException instead of the one that you expect.

If you replace

self.Equals(default(T))

with

EqualityComparer<T>.Default.Equals(obj, default(T))

you should start getting the expected ArgumentException.