What you could test is that ProductDAL.DeleteProduct is called with the correct parameters. This can be accomplished by using dependency injection and mocks!
Sample using Moq as mocking framework:
public interface IProductDal
{
void DeleteProduct(int id);
}
public class MyService
{
private IProductDal _productDal;
public MyService(IProductDal productDal)
{
if (productDal == null) { throw new ArgumentNullException("productDal"); }
_productDal = productDal;
}
public void DeleteProduct(int id)
{
_productDal.DeleteProduct(id);
}
}
Unit test
[TestMethod]
public void DeleteProduct_ValidProductId_DeletedProductInDAL()
{
var productId = 35;
//arrange
var mockProductDal = new Mock<IProductDal>();
var sut = new MyService(mockProductDal.Object);
//act
sut.DeleteProduct(productId);
//assert
//verify that product dal was called with the correct parameter
mockProductDal.Verify(i => i.DeleteProduct(productId));
}