Efficient Unit Test in C#.NET

I consider unit tests one of the most important practices in agile programming.
As the name stated, unit tests check the behaviour of the smaller part of an object what can be a constructor, method or property.

[TestFixture]
public class SomeTest {
  [Test]
  public void CheckDiscountCalculation() {
    Product prod1 = new Product("T-Shirt",125.0d); // description and price
    Product prod2 = new Product("Apple",10.0d);
    Product prod3 = new Product("Pants",50.0d);

    OrderItem item1 = new OrderItem(prod1,1); // product and quantity
    OrderItem item2 = new OrderItem(prod2,1);
    OrderItem item3 = new OrderITem(prod3,1);

    Client client = new Client("Jack");

    Order order = new Order(client);
    order.AddItem(item1);
    order.AddItem(item2);
    order.AddItem(item3);

    decimal discount = order.CalculateDiscount();

    Assert.AreEqual(18.5d, discount);        
  }
}

Although the test example above is perfectly possible you can note that much of this test was spent in initializing the neighbour objects before actually testing the discount calculation.
In real world tests, this can get much more complicated, with several dependencies around the objects. Besides that it can be even more difficult to initialize objects when they access the database internally even through interfaces.

In order to minimize this problem, there are mock frameworks that can be used together with NUnit or your preferred unit testing framework. Basically mock framework can dynamically generate fake objects from class types so that you can focus on testing in your target object.
In the example above, the target is the Order. My preferred Mock framework is Moq due to its heavy usage of delegate notations which avoid using magical strings to configure Mock classes and setup methods.

The above test now can be rewritten like this:

[TestFixture]
public class SomeTest
{
  [Test]
  public void CheckDiscountCalculation()
  {
    Mock prod1Mock = new Mock();
    // Ignores method implementation and returns 125.0d
    prod1Mock.Setup(p=>p.Price).Returns(125.0d); 

    Mock prod2Mock = new Mock();
    prod2Mock.Setup(p=>p.Price).Returns(10.0d);

    Mock prod3Mock = new Mock();
    prod3Mock.Setup(p=>p.Price).Returns(50.0d);


    // product and quantity
    OrderItem item1 = new OrderItem(prod1,1);
    OrderItem item2 = new OrderItem(prod2,1);
    OrderItem item3 = new OrderITem(prod3,1);

    Client client = new Client("Jack");

    Order order = new Order(client);
    order.AddItem(item1);
    order.AddItem(item2);
    order.AddItem(item3);

    decimal discount = order.CalculateDiscount();

    Assert.AreEqual(18.5d, discount);        
  }
}