From Static to Flexible

Back in the “good old days” static classes and singletons where widely used patterns. They could be called from anywhere and you didn’t have to spend a thought on dependency management. Or unit testing. Because most often you couldn’t test code that called methods on those objects. They usually needed config files, databases, web-services, message queues, more files, network shares … just about anything. And none of that plays nice with unit tests. Pity that these days can also be called “earlier this year”…

As a “tame” example of such a static object think of the File class. Now imagine you have a component that uses File.Exists(). How do you test that code? Do you really want to create a file whenever you try to run a test of your component?

Lets look at a sample to show how you can still test that code.

public class MyComponent
{
  public void DoIt(string pathToFile)
  {
    if(File.Exists(pathToFile)) { /* ... */ }
    else { /* ... */ }
  }
}

In order to simulate the situation where the file exists you would have to create said file. That’s not what unit tests are for. They should be almost effortless to write. Wrestling with the file systen in a unit test scenario is definitely not effortless.

A common recommendation is to write a wrapper for the File class which is non-static. That would look like this:

public class FileWrapper
{
  public bool Exists(string path)
  {
    return File.Exists(path);
  }
}

But that’s not the whole story. We need a way to simulate the result of FileWrapper.Exists() or we would just add code but won’t gain anything. To do that we lend ourselves the solution Microsoft used for HttpContext. We introduce a base class with all virtual properties and methods.

public class FileBase
{
  public virtual bool Exists(string path)
  {
    throw new NotImplementedException();
  }
}
public class FileWrapper : FileBase
{
  public override bool Exists(string path)
  {
    return File.Exists(path);
  }
}

Now we change our component to use some instance of FileBase instead of the static File class.

public class MyComponent
{
  private readonly FileBase file;
  public MyComponent(FileBase file)
  {
    this.file = file;
  }
  public void DoIt(string pathToFile)
  {
    if(this.file.Exists(pathToFile)) { /* ... */ }
    else  { /* ... */ }
  }
}

We can use Moq or some hand-crafted test double that implements FileBase to simulate the existance or non-existance of said file very easily.

To make things a bit nastier imagine that MyComponent is used throughout a large legacy codebase. You just changed the signature of it’s constructor. Now you would have to change all calls to that constructor immediately or the system will be broken. That’s not an option.

But if we add a constructor overload that delegates to the constructor we just created and uses the default implementation for FileBase we can leave all calls to the default constructor in place but still have the improved testability of the constructor that follows the Dependency Injection Pattern.

public class MyComponent
{
  private readonly FileBase file;
  public MyComponent() : this(new FileWrapper())
  {
  }
  public MyComponent(FileBase file)
  {
    this.file = file;
  }
  // more code
}

I know that this solution is not pure. The DI Pattern advocates the usage of one constructor only that takes all the dependencies of a class. I prefer that as well. But “legacy enterprise systems” are rarely what I’d call clean and sometimes you have to take some dirty intermediate steps towards clean solutions. With the technique outlined above you can improve such systems one step at a time. You can control how far these improvements are visible by adding or removing constructors that create default implmentations for their dependencies and replacing calls to these default constructors one at a time. Rome wasn’t built in a day…

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