Open closed example, step 1

FilterTest.cs

[TestFixture]
class FilterTest
{
	[Test]
	public void filterByBlue_return_2()
	{
		// arrange
		ProductFilter filter = new ProductFilter();
		IList<Product> products = BuildProducts();

		// act
		var result = filter.ByColor(products, ProductColor.Blue);

		// assert
		Assert.That(result.Count(), Is.EqualTo(2));
		Assert.That(result, Has.All.Matches<Product>(x => x.Color == ProductColor.Blue));
	}

	[Test]
	public void filterBySmall_return_2()
	{
		// arrange
		ProductFilter filter = new ProductFilter();
		IList<Product> products = BuildProducts();

		// act
		var result = filter.BySize(products,  ProductSize.Small);

		// assert
		Assert.That(result.Count(), Is.EqualTo(2));
		Assert.That(result, Has.All.Matches<Product>(x => x.Size == ProductSize.Small));
	}

	[Test]
	public void filterByBlueAndSmall_return_1()
	{
		// arrange
		ProductFilter filter = new ProductFilter();
		IList<Product> products = BuildProducts();

		// act
		var result = filter.ByColorAndSize(products, ProductColor.Blue, ProductSize.Small);

		// assert
		Assert.That(result.Count(), Is.EqualTo(1));
		Assert.That(result, Has.All.Matches<Product>(x => x.Size == ProductSize.Small));
	}

	private IList<Product> BuildProducts()
	{
		return new List<Product> { 
			new Product(ProductColor.Blue, ProductSize.Small), 
			new Product(ProductColor.Yellow, ProductSize.Small), 
			new Product(ProductColor.Yellow, ProductSize.Medium), 
			new Product(ProductColor.Red, ProductSize.Large), 
			new Product(ProductColor.Blue, ProductSize.ReallyBig)
		};
	}
}