Dependency Injection with TypeMock

Listing 2 – the simplest solution

public void ValidateThatNameIsOk(string name)

{

}

 

 

Now lets add a failing case (we must mock out the MessageBox or we will halt the test)

Listing 3 – another test.

Part 2 – More Features Request

Our application is a success and we managed to deliver on time (because we didn’t write unneeded code). After some time our customer asked for a new feature: “run the application in batch mode”. Now we cannot use MessageBox.Show() any more. Now we need to write the message to the console (or a file).

Lets start with the test*.

Listing 5
[Test,VerifyMocks]
public void ValidateThatNameIsOk_FailsAndWritesToConsole_WhenNameIsEmpty()
{
    // Record Mocks
    using (RecordExpectations recorder = RecorderManager.StartRecording())
    {
        Console.WriteLine(“A Name must be entered”);
        recorder.CheckArguments();
    }
    SomeApplication classUnderTest = new SomeApplication();
    classUnderTest.ValidateThatNameIsOk(“”);
}

This test fails, of course. One way to solve this problem is to use an if statement and a property to control the output mechanism.

 

 

Listing 6 – first solution

private bool useConsole = false;

public  bool UseConsole

{

    set { useConsole = value; }

}

public void ValidateThatNameIsOk(string name)

{

    if (name.Length==0)

    {

        if (useConsole)

            Console.WriteLine(“A Name must be entered”);

        else

            MessageBox.Show(“A Name must be entered”);

    }

}

 

 

Lets refactor this. The Console/MessageBox decision needs to be performed in many places. Lets extract a class Message to do this for us.

 

Listing 7 – the Message Class

public class Message
{

    private static bool useConsole = false;

    public  static bool UseConsole

    {

        set { useConsole = value; }

    }

 

    public static void Show(string message)

    {

        if (useConsole)

            Console.WriteLine(“A Name must be entered”);

        else

            MessageBox.Show(“A Name must be entered”);      

    }

}

 

and here is the validation method.

private static MessengerType messenger = MessengerType.MessageBox;

public static MessengerType Messenger

{

    set { messenger = value; }

}

 

public static void Show(string message)

{

    switch (messenger)

    {

        case MessengerType.MessageBox:

            MessageBox.Show(message);

            break;

        case MessengerType.Console:

            Console.WriteLine(message);

            break;

        case MessengerType.File:

            File.AppendText(message);

            break;

    }

}

public interface IMessage

{

    void Show(string message);

}

public class MessageBoxMessage : IMessage

{

    public void Show(string message)

    {

        MessageBox.Show(message);      

    }

}

public class ConsoleMessage : IMessage

{

    public void Show(string message)

    {

        Console.WriteLine(message);

    }

}

public class Message

{

    private static IMessage messenger = new MessageBoxMessage();

    public static IMessage Messenger

    {

        set { messenger = value; }

    }

 

    public static void Show(string message)

    {

        messenger.Show(“A Name must be entered”);      

    }

}

 

Take a look at the code. We have reached the Dependency Injection pattern. Here is how we inject the correct messenger.