How do I unit test an adapter class?

Question

Suppose I have the following classes from a third-party library:

public class ThirdPartyType { ... }

public class ThirdPartyFunction
{
    public ThirdPartyType DoSomething() { ... }
}

The implementation details are not important, and they are actually outside of my control for this third-party library.

Suppose I write an adapter class for ThirdPartyFunction :

public class Adapter
{
    private readonly ThirdPartyFunction f;

    public Adapter()
    {
        f = new ThirdPartyFunction();
    }

    public string DoSomething()
    {
        var result = f.DoSomething();

        // Convert to a type that my clients can understand
        return Convert(result);
    }

    private string Convert(ThirdPartyType value)
    {
        // Complex conversion from ThirdPartyType to string
        // (how do I test this private method?)
        ...
    }
}

How can I test that my implementation of Convert(ThirdPartyType) is correct? It's only needed by the Adapter class, which is why it's a private method.

Answer 1

I would recommend extracting the code to a seprate class and then testing that class. Although it is only used by this Adapter it shouldn't be the responsibility of the adapter to do the conversion as well (in keeping with the Single Responsibility Principle).

By extracting it out it gives you the ability to test the converter in isolation from the third party code.

If the converter does not require any state you could also make it a static class and then reference it directly in your adapter without the need for registering it with dependency injection.

If you think about it, the adapter doesn't need testing (as it is just a wrapper) but the converter does - so extracting it to another class makes sense to allow it to be tested, even if it does mean another class in the code.

In addition, extracting the converter to a separate class means that if the ThirdPartyType or the format of your string changes then you can make the changes without affecting the Adapter implementation.

Answer 2

If you change your Adapter class to allow the ThirdPartyFunction f to be passed to it, then you can use a mock version of it in your test class. this will allow you to test the Convert function.

public class Adapter
{
  private readonly ThirdPartyFunction f;

  // pass the function to the constructor when creating the adapter
  public Adapter(ThirdPartyFunction inputF)
    {
      f = inputF;
    }

  public string DoSomething()
  {
    var result = f.DoSomething();

    // Convert to a type that my clients can understand
    return Convert(result);
  }

  private string Convert(ThirdPartyType value)
  {
    // Complex conversion from ThirdPartyType to string
    // (how do I test this private method?)
    ...
  }
}