Validation specification testing for c# domain entities

I have just been revisiting fluent configuration in the context of writing a fluent tester for validation of my domain entities. In that post, I wrote my test assertions as:

   ValidMO.ImageBanner.ShouldNotBeNull();
   ValidMO.ImageBanner.IsValid().ShouldBeTrue();
   new ImageBanner().SetupWithDefaultValuesFrom(ValidMO.ImageBanner).IsValid().ShouldBeTrue();
   new ImageBanner { Name = "" }.SetupWithDefaultValuesFrom(ValidMO.ImageBanner).IsValid().ShouldBeFalse();

This is okay and I have had luck teaching it. But, it feels long-winded for what I really want to do:

• check that my fake is valid
• check validations for each property (valid and invalid)

Today, I was looking at Fluent Nhibernate and noticed their persistence specification testing. This looks much more expressive:

	[Test]
	public void CanCorrectlyMapEmployee()
	{
	    new PersistenceSpecification<Employee>(session)
	        .CheckProperty(c => c.Id, 1)
	        .CheckProperty(c => c.FirstName, "John")
	        .CheckProperty(c => c.LastName, "Doe")
	        .VerifyTheMappings();
	}

How about this then for a base test:

	Test.ValidationSpecification<ImageBanner>(ValidMO.ImageBanner)
		.CheckPropertyInvalid(c => c.Name, "")
		.Verify();

This test makes some assumptions:

• It will call IsValid() method on the entity
• It will need to make assertion with your current test framework (fine, we do that in storyq)

You can see that I prefer the static constructor rather than using new.

There are obviously, a range of syntax changes I could make here that would mimic the validation attributes. For example:

	Test.ValidationSpecification<ImageBanner>(ValidMO.ImageBanner)
		.CheckPropertyNotEmpty(c => c.Name)
		.CheckPropertyMandatory(c => c.Name)
		.Verify();

Because the .CheckProperty would be extension method, you could easily add then as you go for your validations. Let’s start with that because that’s all I need for now – we will want to be able to change the callable IsValid method. Fluent nhibernate also passes in a IEqualityComparer that makes me wonder if a mechanism like this could be useful – it certainly looks cool!