Testing code that depends on the corporate library, even if it does not provide interfaces?

Question

Maybe I'm showing that I don’t understand dependency injection and testing, but I don’t understand how using dependency injection with classes that don’t implement interfaces helps me when testing?

For example, the Enterprise Library 5.0 documentation talks about using a Unity container to create instances. It says it helps "testability: it is trivial to isolate classes from dependencies using the dependency injection style." MSDN

How can I use this in my unit tests? In their example, there is a constructor with parameters as classes, not interfaces:

public class TaxCalculator 
{
  private ExceptionManager _exceptionManager;
  private LogWriter _logWriter;

  public TaxCalculator(ExceptionManager em, LogWriter lw) 
  {
    this._exceptionManager = em;
    this._logWriter = lw;
  }
}

Answer 1

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Answer 2

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public abstract class LogWriter
{
    public abstract void Write(string message);
}

, :

Mock<LogWriter> logWriter = new Mock<LogWriter>();
TaxCalculator calc = new TaxCalculator(logWriter.Object);

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Answer 3

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