Dependency injection design pattern

Normally when designing some parts of the application, one classes is always depends on the other classes. For example, business logic classes (BL) are depend on data access classes (DAL). This dependence can be in very different ways, for example, BL can call static methods of DAL, or create instance of DAL every time it needs it… But in this case it’s can be difficulties to test business logic with Unit tests because it’s difficult or even impossible to create mocks to the database. To solve this problem you can use Dependency injection pattern. Here is example of how I am using it. Firstly create our classes for DAL:

public class ProductDAL
    {
        public virtual List<Product> GetProducts()
        {
            // connect to DB and get products
            return new List<Product>();
        }
    }

And for BL:

public class ProductBL
    {
        private readonly ProductDAL dal;

        public ProductBL(ProductDAL dal)
        {
            this.dal = dal;
        }

        public List<Product> GetProducts()
        {
            // Get products, make some manipulation with the products,
            // e.g. sorting, filtering...
            return dal.GetProducts();
        }
    }

For parameter in constructor in BL I passed instance of DAL, it’s Constructor injection. And it’s very easy to test this calls with Rhino mocks.

[TestFixture]
    public class ProductBLTests
    {
        [Test]
        public void GetProductsTest()
        {
            MockRepository mocks = new MockRepository();
            ProductDAL dal = (ProductDAL)mocks.StrictMock(typeof(ProductDAL));
            using (mocks.Record())
            {
                List<Product> expectedProducts = new List<Product>();
                Product p = new Product();
                p.Name = "Dell Pc";
                p.Id = 333;
                expectedProducts.Add(p);

                Expect.Call(dal.GetProducts()).Return(expectedProducts);
            }

            using (mocks.Playback())
            {
                ProductBL bl = new ProductBL(dal);
                List<Product> products = bl.GetProducts();
                Assert.AreEqual(1, products.Count);
            }

        }
    }

Some developers can say than Dependency Injection is a not good pattern for database layer and there are some another patterns for this purpose, but I like this solution, and successfully using it.