如何重构遗留代码以更好地适合单元测试

Question

I have this legacy code I inherited. The problem is I have this read-only class that handles some parsing logic.

public class ParsedClass
{
    public int ID 
    { 
       get { return _id; }
    }
    private readonly int _id;

    public bool IsMale
    { 
       get { return _isMale; }
    }
    private readonly bool _isMale;

    public ParsedClass(byte[] payload)
    {
        // parsing logic
        // assign properties etc etc 
    }
}

This code then gets passed to some business logic that depends on this parsed class. And is used something like.

public class BusinessLogic
{
    public int ProcessingLogic(ParsedClass obj)
    {
        // do some processing stuff 
        User acct = DataAccess.GetUser(obj.ID);
        if(obj.IsMale)
            // do some other business logic 
    }
}

So as you can see the class is tightly coupled with the business logic. Since the parsing class is closed from the outside. The only way to unit test some functions is to create a payload that has the data I need. How could I best refactor this to make it more open for unit testing?

Initially, I thought about opening up the class to modification from the outside. This would let me assign the values I need without having to craft a specific payload for each scenario. But, I have been reading around stack, and it seems like doing this would be bad design.

Answer 1

Consider adding protected or internal constructor to your ParsedClass :

public class ParsedClass
{
    // your legacy stuff

    protected ParsedClass(int id, bool isMale)
    {
        _id = id;
        _isMale = isMale;
    }
}

Then you can add opened inherited class for tests ( with internal constructor of the base class you needn't this, but your tests must be in the same assembly however ):

public class TestParsedClass : ParsedClass
{
    public TestParsedClass(int id, bool isMale) : base(id, isMale)
    {
    }
}

Now you can test your BusinessLogic class from wherever:

new BusinessLogic().ProcessingLogic(new TestParsedClass(5, true));

---

Another way is to declare interface for your ParsedClass :

public interface IParsedClass
{
    public int ID { get; }

    public bool IsMale { get; }
}

Then refactor your classes like this:

public class ParsedClass : IParsedClass
{
    // your legacy stuff
}

public class BusinessLogic
{
    public int ProcessingLogic(IParsedClass obj)
    {
        // your legacy stuff
    }
}

Now you can create some test class implementing IParsedClass like this:

public class TestParsedClass : IParsedClass
{
    public int ID { get; set; }

    public bool IsMale { get; set; }
}

and use it in your BusinessLogic unit tests.

This approach gives you bonus advantage - your ProcessingLogic method become more flexible now.

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Hope this would be suitable for your project.