Replacing Switch Statements with ChainStrategy

Problem Statement

This article is a quick introduction and example of implementing a strategy via the ChainStrategy library. Please see the official documentation for more information and samples.

Sometimes we have use cases in our code that lead to natural logical branches where a process separates into sub-processes and may return a common response.

In the example below, a service implementing a use-case runs conditional logic depending on what the request is doing.

public class TrackingService
{
    private readonly IUpsDependency _upsDependency;
    private readonly IFedexDependency _fedExDependency;
    private readonly IUspsDependency _uspsDependency;

    public TrackingService(
        IUpsDependency upsDependency,
        IFedexDependency fedExDependency,
        IUspsDependency uspsDependency
    )
    {
        _upsDependency = upsDependency;
        _fedexDependency = fedexDependency;
        _uspsDependency = uspsDependency;
    }

    public TrackingResponse TrackPackage(TrackingRequest request)
    {
        if (request.trackingNumber == upsCode)
        {
            return TrackUps();
        }
        else if (request.trackingNumber == fedExCode)
        {
            return TrackFedEx();
        }

        return TrackUsps();
    }

    private TrackingResponse TrackUps()
    {
        // Ups implementation
    }

    private TrackingResponse TrackFedEx()
    {
        // FedEx implementation
    }

    private TrackingResponse TrackUsps()
    {
        // Usps implementation
    }
}

This code, while workable, is sub-optimal. The problems with this code sample are:

• We have private methods when we should have public ones.
• Unit testing requires mocking dependencies which may not be used

A strategy here would allow us to break each condition into an individual handler. This would transform our use case into smaller chunks that each fulfill a single responsibility. This allows us to easily unit test each handler without having to stub and mock dependencies that each condition does not rely on.

Enter the Strategy

The strategy pattern is a simple way to employ an object-oriented switch statement. You can divide a problem into multiple handlers, each with its specific dependencies. This directly leads to easier automated testing because each test can focus on a single purpose.

A Strategy is best used when:

• You have conditional logic that has complex processes
• Each process shares common input and output

The Strategy pattern is a low-usage pattern. It does not need to replace every switch statement in your code.

Every strategy has a dedicated request and response object.

public class TrackingResponse
{
    // response properties
}

public class TrackingRequest : IStrategyRequest<TrackingResponse>
{
    // request properties
}

For each possible logical condition, we will define a custom handler that only uses the required dependencies to fulfill each logical branch.

public class FedexTrackingHandler : IStrategyHandler<TrackingRequest, TrackingResponse>
{
    private readonly IFedexDependency _fedExDependency;

    public FedexTrackingHandler(IFedexDependency fedExDependency)
    {
        _fedexDependency = fedexDependency;
    }

    public async Task<TrackingResponse> Handle(TrackingRequest request, CancellationToken cancellationToken)
    {
        // implementation
    }
}

Testing each handler is a lot easier than trying to write tests for a larger service because you are mocking and testing fewer dependencies and logical branches.

[TestClass]
public class FedExTrackingHandlerTests
{
    private readonly Mock<IFedexDependency> _fedex;

    public FedexTrackingHandlerTests()
    {
        _fedex = new Mock<IFedexDependency>();
    }

    [TestMethod]
    public void Handler_Condition_IsCorrect()
    {
        // implement tests
    }
}

Finally, we need to create a profile which will be used to determine what handler to use.

public class TrackingProfile
{
    public TrackingProfile()
    {
        AddStrategy<FedExTrackingHandler>(x => x.TrackingNumber == fedExCode);
        AddStrategy<UpsTrackingHandler>(x => x.TrackingNumber == upsCode);
        AddStrategy<UspsTrackingHandler>(x => x.TrackingNumber == uspsCode);
    }
}

Utilizing our new strategy from a controller is easy:

[ApiController]
[Route("[controller]")]
public class PackageController : ControllerBase
{
    private readonly IStrategyFactory _factory;

    public PackageController(IStrategyFactory factory)
    {
        _factory = factory;
    }

    [HttpGet("{trackingNumber:int}")]
    public async Task<IActionResult> TrackPackage(int trackingNumber)
    {
        var response = await _factory.Execute(new TrackingRequest(trackingNumber));

        return Ok(response);
    }
}

Conclusion

Use cases with complex conditional logic tend to lead to large services that tend to have lots of dependencies and private methods. This results in a frustrating testing experience when dealing with lots of mocking and brittle conditional testing. A Strategy may help to alleviate this situation by breaking up a problem into smaller chunks that can be implemented and tested in isolation from other code blocks. This leads to a stronger code base that is easier to maintain in the long run.