Xunit.Combinatorial: A Simple Way to Test Combinations

Sugumar Panneerselvam
5 min readOct 20, 2024

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Photo by Towfiqu barbhuiya on Unsplash

Introduction to Combinatorial Testing

Combinatorial testing is a powerful technique used to test all possible combinations of inputs to a system, ensuring comprehensive coverage. In complex systems, certain features or functions may behave differently depending on various input conditions. Testing all combinations manually or writing individual test cases for each combination can be both tedious and error-prone. This is where combinatorial testing shines — it automates the process, making it efficient to test all combinations.

In a previous article, I explored Instancio for Java, which simplifies generating random test data. While exploring similar options in the .NET world, I found Xunit.Combinatorial. It allows .NET developers to quickly and efficiently test multiple combinations of input parameters in a single test method, improving code quality and reliability. Like Instancio, Xunit.Combinatorial brings automation and ease to the testing process, but it focuses specifically on combinatorial testing.

Getting Started with Xunit.Combinatorial

To get started, you first need to install the Xunit.Combinatorial NuGet package. You can do this using the NuGet Package Manager or by running the following command:

dotnet add package Xunit.Combinatorial

Once installed, you can begin using the CombinatorialData attribute to write tests that run against multiple combinations of parameters.

For example, let’s take a scenario from the finance domain:

public class LoanApprovalTests
{
[Theory]
[CombinatorialData]
public void TestLoanApproval(
[CombinatorialValues(10000, 50000, 100000)] decimal loanAmount,
[CombinatorialValues(3, 5, 10)] int loanTermInYears,
[CombinatorialValues("Excellent", "Good", "Fair")] string creditScore)
{
// Arrange
var loanService = new LoanService();

// Act
bool isApproved = loanService.ApproveLoan(loanAmount, loanTermInYears, creditScore);

// Assert
Assert.True(isApproved);
}
}

In this example, the test checks the loan approval process based on three variables: loanAmount, loanTermInYears, and creditScore. Xunit.Combinatorial will automatically generate all possible combinations of these parameters, running multiple test cases to ensure the loan approval logic works across different scenarios. This generates 27 test cases (3 amounts × 3 terms × 3 credit scores).

Using [CombinatorialRange] for Efficient Testing

The [CombinatorialRange] attribute offers a convenient way to define a range of numeric values to be used as parameters in your tests. This is particularly useful when testing scenarios that require testing a broad range of inputs without manually specifying each value.

For example, consider a test for a financial system where you want to evaluate loan interest calculations over a range of years and amounts:

public class InterestCalculationTests
{
[Theory]
[CombinatorialData]
public void TestInterestCalculation(
[CombinatorialValues("Home Loan", "Car Loan")] string loanType,
[CombinatorialRange(5000, 50000, 10000)] decimal loanAmount, // Test loan amounts in steps of 10,000
[CombinatorialRange(1, 5)] int loanTermInYears) // Test loan terms between 1 and 5 years
{
// Arrange
var interestCalculator = new InterestRateCalculator();

// Act
decimal interest = interestCalculator.CalculateInterest(loanType, loanAmount, loanTermInYears);

// Assert
Assert.InRange(interest, 1, 15); // Assuming interest rates between 1% and 15%
}
}

In this example:

  • [CombinatorialRange(5000, 50000, 10000)] generates values from 5000 to 50000 in steps of 10000.
  • [CombinatorialRange(1, 5)] generates loan terms between 1 and 5 years.

This allows you to efficiently test combinations of loan types, amounts, and terms without manually specifying each possible value, ensuring you can cover a broad range of scenarios with minimal effort.

Exploring [PairwiseData] for Efficient Testing

While CombinatorialData allows for testing all possible combinations of parameters, there are cases where testing every single combination may not be necessary or could result in an excessive number of test cases. This is where the [PairwiseData] attribute can be helpful.

Pairwise testing ensures that for each pair of parameters, every combination of their values is tested at least once. This significantly reduces the number of test cases while still covering most potential interactions between the parameters.

Here’s an example of how you can use [PairwiseData] in your tests:

public class LoanProcessingTests
{
[Theory]
[PairwiseData]
public void TestLoanProcessing(
[CombinatorialValues("Small Business", "Personal", "Mortgage")] string loanType,
[CombinatorialValues("Approved", "Pending", "Rejected")] string applicationStatus,
[CombinatorialValues("High", "Medium", "Low")] string creditRisk)
{
// Arrange
var loanProcessor = new LoanProcessor();

// Act
bool result = loanProcessor.ProcessLoan(loanType, applicationStatus, creditRisk);

// Assert
Assert.True(result);
}
}

In this example, [PairwiseData] ensures that every combination of pairs of loanType, applicationStatus, and creditRisk is covered, leading to fewer test cases but still providing meaningful coverage.

When to Use [PairwiseData]

  • Performance: If testing every combination would result in too many test cases, pairwise testing provides a more efficient alternative.
  • Coverage: Though it reduces the total number of test cases, it still ensures that important interactions between parameters are thoroughly tested.
  • Trade-Off: Use [PairwiseData] when full combinatorial testing is overkill, but you still want meaningful coverage of interactions.

Benefits of Xunit.Combinatorial

The primary benefit of Xunit.Combinatorial is the ability to easily test all possible combinations of parameters in a systematic way. This approach offers several key advantages:

  • Increased Coverage: Combinatorial testing ensures that every possible combination of inputs is tested, which helps catch edge cases or unexpected interactions that might not be caught with traditional testing methods.
  • Reduced Code Duplication: Instead of writing separate test methods for each combination of input parameters, you can handle it all in a single test, leading to cleaner and more maintainable code.
  • Improved Efficiency: Xunit.Combinatorial makes it easy to add new parameters or values to tests without duplicating test code. This reduces the time spent writing tests while increasing the robustness of your testing process.

Compared to traditional testing, where each test case is written manually, Xunit.Combinatorial provides an automated and scalable approach, allowing developers to focus on writing meaningful test logic rather than managing test case permutations.

Conclusion

In conclusion, Xunit.Combinatorial simplifies the process of testing multiple combinations of inputs, helping ensure that your code behaves correctly under all conditions. By reducing the need for duplicate tests and improving overall test coverage, it enhances both the efficiency and the reliability of your testing framework.

In a complex codebase, having the ability to automate combinatorial testing with tools like Xunit.Combinatorial is invaluable. It allows you to confidently test edge cases and catch potential issues that would otherwise be missed. Just as Instancio helps Java developers automate test data generation, Xunit.Combinatorial offers a similar benefit to .NET developers by automating combinatorial tests — making it an essential tool for your .NET testing toolkit.

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