How to make an existing public API testable for external programmers using it?

Question

I have a C# public API that is used by many third-party developers that have written custom applications on top of it. In addition, the API is used widely by internal developers.

This API wasn't written with testability in mind: most class methods aren't virtual and things weren't factored out into interfaces. In addition, there are some helper static methods.

For many reasons I can't change the design significantly without causing breaking changes for applications developed by programmers using my API. However, I'd still like to give internal and external developers using this API the chance to write unit tests and be able to mock the objects in the API.

There are several approaches that come to mind, but none of them seem great:

1. The traditional approach would be to force developers to create a proxy class that they controlled that would talk to my API. This won't work in practice because there are now hundreds of classes, many of which are effectively strongly typed data transfer objects that would be a pain to reproduce and maintain.

2. Force all developers using the API that want to unit test it to buy TypeMock. This seems harsh to force people to pay $300+ per developer and potentially require them to learn a different mock object tool than what their used to.

3. Go through the entire project and make all the methods virtual. This would allow mock-ing of objects using free tools like Moq or Rhino Mocks, but it could potentially open up security risks for classes that were never meant to be derived from. Additionally this could cause breaking changes.

4. I could create a tool that given an input assembly would output an assembly with the same namespaces, classes, and members, but would make all of the methods virtual and it would make the method body just return the default value for the return type. Then, I could ship this dummy test assembly each time I released an update to the API. Developers could then write tests for the API against the dummy assembly since it had virtual members that are very mock-able. This might work, but it seems a bit tedious to write a custom tool for this and I can't seem to find an existing one that does it well (especially that works well with generics). Furthermore, it has the complication that it requires developers to use two different assemblies that could possibly go out of date.

5. Similar to #4, I could go through every file and add something like "#ifdef UNITTEST" to every method and body to do the equivalent of what a tool would do. This doesn't require an external tool, but it would pollute the codebase with a lot of ugly "#ifdef"'s.

Is there something else that I haven't considered that would be a good fit? Does a tool like what I mentioned in #4 already exist?

Again, the complicating factor is that this is a rather large API (hundreds of classes and ~10 files) and has existing applications using it which makes it hard to do drastic design changes.

There have been several questions on Stack Overflow that were generic about retrofitting an existing application to make it testable, but none seem to address the concerns I have (specifically in the context of a widely used API with many third-party developers). I'm also aware of "Working Effectively With Legacy Code" and think it has good advice, but I am looking for a specific .net approach given the constraints mentioned above.

UPDATE: I appreciate the answers so far. One that Patrik Hägne brought up is "why not extract interfaces?" This indeed works to a point, but has some problems such as the existing design has many cases where we take expose a concrete class. For example:

public class UserRepository 
{ 
    public UserData GetData(string userName) 
    {
        ...
    } 
}

Existing customers that are expecting the concrete class (e.g. "UserData") would break if they were given an "IUserData."

Additionally, as mentioned in the comments there are cases where we take in a class and then expose it for convenience. This could cause problems if we took in an interface and then had to expose it as a concrete class.

The biggest challenge to a significant rewrite or redesign is that there is a huge investment in the current API (thousands of hours of development and probably just as much third party training). So, while I agree that a better SOLID design rewrite or abstraction layer (that eventually could become the new API) that focused on items like the Interface Separation Principle would be a plus from a testability perspective, it'd be a large undertaking that probably can't be cost justified at the present time.

We do have testing for the current API, but it is more complicated integration testing rather than unit-testing.

Additionally, as mentioned by Chad Myers, this is question addresses a similar problem that the .NET framework itself faces in some areas.

I realize that I'm probably looking for a "silver bullet" here that doesn't exist, but all help is appreciated. The important part is protecting the huge time investments by many third party developers as well as the huge existing development to create the current API.

All answers, especially those that consider the business side of the problem, will be carefully reviewed. Thanks!

Answer 1

What you're really asking is, "How do I design my API with SOLID and similar principles in mind so my API plays well with others?" It's not just about testability. If your customers are having problems testing their code with yours, then they're also having problems WRITING/USING their code with yours, so this is a bigger problem than just testability.

Simply extracting interfaces will not solve the problem because it's likely your existing class interfaces (what the concrete classes expose as their methods/properties) aren't design with Interface Segregation Principle in mind, so the extracted interface would have all sorts of problems (some of which you mentioned in comment to a previous answer).

I like to call this the IHttpContext problem. ASP.NET, as you know, is very difficult to test around or with due to the "Magic Singleton Dependency" problem of HttpContext.Current. HttpContext is not mockable without fancy tricks like what TypeMock uses. Simply extracting an interface of HttpContext is not going to help that much because it's SO huge. Eventually, even IHttpContext would become a burden to test with so much so that it's almost not worth doing any more than trying to mock HttpContext itself.

Identifying object responsibilities, slicing up interfaces and interactions appropriately, and designing with Open/Closed Principle in mind is not something you and try to force/cram into an existing API designed without these principles in mind.

I hate to leave you with such a grim answer, so I'll give you one positive suggest: How's about YOU take all the grief on behalf of your customers and make some sort of service/facade layer over top of your old API. This service layer will have to deal with the minutiae and pain of your API, but will present a nice, clean, SOLID-friendly public API that your customers can use with much less friction.

This also has the added benefit of allowing you to slowly replace parts of your API and eventually make it so your new API isn't just a facade, it IS the API (and the old API is phased out).