Property-Based Testing with FsCheck

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IntroductionIntroduction
Understanding Property-Based TestingUnderstanding Property-Based Testing
Setting Up FsCheck in Your C# ProjectSetting Up FsCheck in Your C# Project
Defining Properties for Your TestsDefining Properties for Your Tests
Effective Usage of QuickCheckEffective Usage of QuickCheck
BDD and Property-Based TestingBDD and Property-Based Testing

Property-Based Testing with FsCheck

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crj4wm

4 min read

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Property-based testing, a powerful technique for verifying code correctness, has gained popularity in functional programming and is now accessible to C# developers through FsCheck, a versatile library that generates random test cases to validate properties of your code.

Understanding Property-Based Testing

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Property-based testing is a software testing approach that focuses on verifying general properties or invariants of a system, rather than specific examples

. Unlike traditional example-based testing, which relies on predefined inputs and expected outputs, property-based testing generates a wide range of random inputs to validate that certain properties hold true for all cases

. This approach helps uncover edge cases and unexpected behaviors that might be missed in conventional testing

. By defining invariants such as "reversing a list twice should result in the original list," developers can create more robust and comprehensive test suites

. Property-based testing is particularly useful for complex data structures, algorithms, and scenarios where exhaustive testing of all possible inputs is impractical

. While it requires more initial effort to define properties and can be slower to execute, property-based testing offers increased confidence in code correctness and can reveal subtle inconsistencies that are difficult to detect with example-based testing alone

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Setting Up FsCheck in Your C# Project

To set up FsCheck in your C# project, start by installing the FsCheck NuGet package using your preferred package manager. For integration with popular testing frameworks, you can install additional packages like FsCheck.xUnit or FsCheck.NUnit

. Once installed, add the necessary using statements to your test files:


csharp
using FsCheck;
using FsCheck.Xunit; // If using xUnit

To write property-based tests, use the [Property] attribute for xUnit or define properties as functions returning bool

. For example:


csharp
[Property]
public bool RevRevIsOrig(List xs) => 
    xs.Reverse().Reverse().SequenceEqual(xs);

You can run these tests using your usual test runner. For more complex scenarios, use Prop.ForAll to define properties with custom generators or conditions

. Remember to configure FsCheck as needed, adjusting parameters like the number of test cases or enabling verbose output for debugging

. With this setup, you can start leveraging FsCheck's powerful property-based testing capabilities in your C# projects.

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Defining Properties for Your Tests

When defining properties for your tests using FsCheck in C#, you need to focus on specifying invariants or characteristics that should hold true for your code under various inputs. Here are some key approaches and examples for defining effective properties:

Reversibility Properties:
These properties check if applying an operation and then its inverse returns the original value. For example, testing a serialization function:


csharp
[Property]
public bool SerializeDeserializeIsIdentity(MyClass obj)
{
    var serialized = Serialize(obj);
    var deserialized = Deserialize(serialized);
    return obj.Equals(deserialized);
}

Consistency Properties:
These ensure that different ways of performing the same operation yield consistent results:


csharp
[Property]
public bool AdditionIsConsistent(int x, int y)
{
    return (x + y == y + x) && (x + y == Add(x, y));
}

Invariant Properties:
These check that certain conditions always hold after an operation:


csharp
[Property]
public bool ListLengthInvariant(List list, int element)
{
    var newList = list.Append(element).ToList();
    return newList.Count == list.Count + 1;
}

Idempotence Properties:
These verify that applying an operation multiple times has the same effect as applying it once:


csharp
[Property]
public bool SortIsIdempotent(List list)
{
    var sortedOnce = list.OrderBy(x => x).ToList();
    var sortedTwice = sortedOnce.OrderBy(x => x).ToList();
    return sortedOnce.SequenceEqual(sortedTwice);
}

Commutativity Properties:
These check if the order of operations doesn't affect the result:


csharp
[Property]
public bool MultiplicationIsCommutative(int x, int y)
{
    return x * y == y * x;
}

When defining properties, it's crucial to consider edge cases and potential limitations. For instance, when dealing with floating-point arithmetic, you might need to use approximate equality comparisons due to rounding errors

. You can also use FsCheck's built-in combinators to create more complex properties. For example, the ==> operator allows you to specify preconditions:


csharp
[Property]
public Property DivisionProperty()
{
    return Prop.ForAll((x, y) => y != 0 ==> (x / y) * y == x);
}

This property only holds when the divisor (y) is not zero

. For more complex scenarios, you can use FsCheck's Classify and Collect methods to gather statistics about your test data distribution:


csharp
[Property]
public Property AbsoluteValueProperty(int x)
{
    return (Math.Abs(x) >= 0)
        .Classify(x > 1000, "large")
        .Classify(x < -1000, "small")
        .Collect($"Number of digits: {Math.Abs(x).ToString().Length}");
}

This not only tests the property but also provides insights into the distribution of test data

. Remember, the goal is to express general truths about your code that should hold for all (or most) inputs. By thinking in terms of these universal properties, you can create more robust tests that catch a wider range of potential issues compared to traditional example-based testing

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Effective Usage of QuickCheck

When using Prop.ForAll.QuickCheck in FsCheck with C#, there are several key strategies to ensure effective property-based testing:

Basic Usage:
The QuickCheck method is a convenient way to run property tests quickly. It's typically used as follows:


csharp
Prop.ForAll((int x, int y) => x + y == y + x)
    .QuickCheck();

This will run 100 tests by default, using randomly generated integers

Throwing on Failure:
For integration with unit testing frameworks, use QuickCheckThrowOnFailure(). This method will throw an exception if the property fails, which most test runners interpret as a test failure:


csharp
Prop.ForAll((int x, int y) => x + y == y + x)
    .QuickCheckThrowOnFailure();

This is particularly useful when running FsCheck tests as part of a larger test suite

Verbose Output:
To get more detailed information about the test runs, use VerboseCheckThrowOnFailure():


csharp
Prop.ForAll((int x, int y) => x + y == y + x)
    .VerboseCheckThrowOnFailure();

This will print the arguments for each test, which can be helpful for debugging or understanding the distribution of test cases

Custom Configurations:
You can customize the test run by passing a Config object:


csharp
var config = Config.Quick.WithMaxTest(1000).WithQuietOnSuccess(true);
Prop.ForAll((int x, int y) => x + y == y + x)
    .Check(config);

This example increases the number of tests to 1000 and suppresses output on success

Labeling and Classification:
Use Label and Classify to gather information about your test cases:


csharp
Prop.ForAll((int x, int y) =>
    (x + y == y + x)
    .Label($"x: {x}, y: {y}")
    .Classify(x > y, "x > y")
    .Classify(x ` operator to specify preconditions:

```csharp
Prop.ForAll((int x, int y) =>
    (y != 0) ==> ((x / y) * y == x - (x % y))
).QuickCheck();

This ensures that the property is only checked when the precondition (y != 0) is met

Custom Generators:
For complex types or specific distributions, you may need to define custom generators:


csharp
var genPositiveInt = Gen.Choose(1, int.MaxValue);
Prop.ForAll(genPositiveInt, x => x > 0)
    .QuickCheck();

This ensures that only positive integers are generated for the test

Integration with Test Frameworks:
When using FsCheck with xUnit or NUnit, you can use attributes to run property tests:


csharp
[Property]
public void CommutativeProperty(int x, int y)
{
    Assert.Equal(x + y, y + x);
}

This allows FsCheck tests to be run alongside traditional unit tests

Shrinking:
FsCheck automatically attempts to shrink counterexamples to simpler ones. For custom types, you may need to implement custom shrinking behavior:


csharp
public class MyTypeArbitrary : Arbitrary
{
    public override Gen Generator => // Custom generator logic

    public override IEnumerable Shrinker(MyType myType) 
        => // Custom shrinking logic
}

. By leveraging these techniques, you can create robust property-based tests that thoroughly exercise your code and uncover edge cases. Remember that the goal is to express general truths about your code that should hold for a wide range of inputs, helping you build more reliable software.

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BDD and Property-Based Testing

Combining Behavior-Driven Development (BDD) scenario testing using SpecFlow with property-based testing via FsCheck can provide a comprehensive testing strategy that leverages the strengths of both approaches. This integration allows teams to benefit from the clear, business-focused scenarios of BDD while also utilizing the power of property-based testing to uncover edge cases and unexpected behaviors. To implement this combined approach:

Define BDD Scenarios:
Use SpecFlow to write Gherkin scenarios that describe the expected behavior of your system from a business perspective. These scenarios serve as executable specifications and help bridge the gap between technical and non-technical team members
4
. For example:


text
Feature: Calculator Operations
Scenario: Adding two numbers
  Given I have entered 50 into the calculator
  And I have entered 70 into the calculator
  When I press add
  Then the result should be 120 on the screen

Implement Step Definitions:
Create step definitions in C# to link the Gherkin scenarios to your actual code. These step definitions can call your application's methods and perform assertions
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.


csharp
[Given(@"I have entered (.*) into the calculator")]
public void GivenIHaveEnteredIntoTheCalculator(int number)
{
    _calculator.Enter(number);
}

[When(@"I press add")]
public void WhenIPressAdd()
{
    _result = _calculator.Add();
}

[Then(@"the result should be (.*) on the screen")]
public void ThenTheResultShouldBeOnTheScreen(int expected)
{
    Assert.Equal(expected, _result);
}

Integrate Property-Based Tests:
Within your step definitions or in separate test methods, use FsCheck to define properties that should hold true for your calculator operations. This allows you to test a wide range of inputs beyond the specific examples in your Gherkin scenarios
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.


csharp
[Property]
public bool AdditionIsCommutative(int x, int y)
{
    return _calculator.Add(x, y) == _calculator.Add(y, x);
}

[Property]
public bool AdditionWithZeroIsIdentity(int x)
{
    return _calculator.Add(x, 0) == x;
}

Combine Scenario and Property Tests:
You can call property-based tests within your SpecFlow step definitions to ensure that properties hold true for specific scenarios as well as for randomly generated inputs
2
.


csharp
[Then(@"addition should be commutative")]
public void ThenAdditionShouldBeCommutative()
{
    Prop.ForAll((x, y) => _calculator.Add(x, y) == _calculator.Add(y, x))
        .QuickCheckThrowOnFailure();
}

Use Tags for Selective Execution:
Utilize SpecFlow tags to categorize your scenarios and selectively run property-based tests for certain features or conditions
4
.


text
@property-test
Scenario: Verify addition properties
  When I perform multiple additions
  Then addition should be commutative
  And addition with zero should be an identity operation

Handle UI Interactions:
For scenarios involving UI interactions, focus on user functionality rather than specific UI elements. Abstract away individual UI actions in your step definitions, allowing you to test through the UI while maintaining readable scenarios
5
.


text
Scenario: User registration
  When I provide my username as "newuser"
  And I submit the registration form
  Then I should see a confirmation message

Apply Hexagonal Architecture:
Consider using Hexagonal Architecture to separate your domain logic from UI concerns. This allows you to run the same scenarios through different interfaces (e.g., UI, API) by swapping out step definition implementations
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.

By combining SpecFlow for BDD scenarios and FsCheck for property-based testing, you create a robust testing strategy that covers both specific business requirements and general properties of your system. This approach helps ensure that your software meets stakeholder expectations while also being resilient to a wide range of inputs and edge cases.