Let me give you an example. You’ve got a class that you want test, but this class somehow depends on an untestable resource like for instance a webservice. You don’t have some kind of dependency inversion to you’re hand and have to solve this problem on your own. A short example:

public class BillingService
{
      public BillingResult DoBilling(BillingData data)
      {
            Bill theBil = MakeBill(data);

            using(BillingClient wc = new BillingClient())
            {
                 wc.DoBilling(theBil);
            }
      }
}

What I mostly recommend in such a situation is

1. Extract or isolate the ugly (Quote from Jeremy Miller) which is the webservice call here.
2. Introduce an interface that shields the class from the webservice call.
3. Make the whole class rely on the abstraction.
4. Use two constructors. I mostly reffered to them as the opened and the closed constructor.

public class BillingService
{
      private IBillingSystem _BillingSystem;

      public BillingService(IBillingSystem billingSystem)
      {
             _BillingSystem = billingSystem;
      }

      public BillingSystem() : this(new BillingSystem())
      {
      }

      public BillingResult DoBilling(BillingData data)
      {
            Bill theBill = MakeBill(data);

            _BillingSystem.DoBilling();
      }
}

public interface IBillingSystem
{
      void DoBilling(Bill theBill);
}

public class BillingSystem : IBillingSystem
{
      public void DoBilling(Bill theBill)
      {
            using(BillingClient wc = new BillingClient())
            {
                 wc.DoBilling(theBill);
            }
      }
}

With that you can easily replace the untestable code with some kind of testdouble like a stub or a mock.

public class BillingServiceTest
{
     [Fact]
     public void DoBilling_ValidBill_BillIsCorrectlySendToBillingSystem()
     {
           BillingSystemStub billingSystem =  new BillingSystemStub();
           BillingService billingService = new BillingService(billingSystem);
           BillingData billingData = MakeValidBillingData();

           billingService.DoBilling(billingData);

           //Do some assertions
           Assert.NotNull(billingSystem.RecievedBill);
     }
}

Today I learned that this is also called POOR MANS DEPENDENCY INJECTION.

P.S.:
Mabe the example is not the best one but I didn’t want to go into something like Foo and Bar again. Besides that it’s only a simplified example. Consider that before throwing eggs and tomatos at me .

P.S.2:
An alternative to this approach which doesn’t require a new interface is “Extract & Override”.