Agent skill

nw-pbt-dotnet

.NET property-based testing with FsCheck, CsCheck, and fsharp-hedgehog frameworks

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SKILL.md

PBT .NET -- FsCheck + CsCheck (C#/F#)

Framework Selection

Framework Language Shrinking Stateful Parallel Choose When
FsCheck C#/F# Type-based No No F#-first projects, or C# needing mature PBT
CsCheck C# PCG-based Yes Yes (linearizability) C# projects needing concurrent testing
fsharp-hedgehog F# Integrated No No F# projects wanting modern integrated shrinking

C# default: CsCheck (better shrinking, stateful + parallel). F# default: FsCheck or fsharp-hedgehog.

Quick Start (FsCheck)

csharp
// C#
using FsCheck; using FsCheck.Xunit;

public class SortProperties
{
    [Property]
    public bool SortPreservesLength(List<int> xs) =>
        xs.OrderBy(x => x).Count() == xs.Count;
}
fsharp
// F#
open FsCheck

let propSortIdempotent (xs: int list) =
    List.sort (List.sort xs) = List.sort xs

Check.Quick propSortIdempotent

Generator Cheat Sheet (FsCheck)

C#

csharp
Arb.Generate<int>()                    // any int
Gen.Choose(0, 100)                     // bounded
Arb.Generate<string>()
Arb.Generate<List<int>>()

// Custom generator
public static Arbitrary<Email> EmailArb() =>
    Arb.From(
        from name in Arb.Generate<NonEmptyString>()
        select new Email($"{name}@example.com")
    );

Arb.Register<MyGenerators>();          // register custom arbitraries

F#

fsharp
Gen.choose (0, 100)
Gen.elements [1; 2; 3]
Gen.oneof [gen1; gen2]
Gen.frequency [(80, gen1); (20, gen2)]
Gen.listOf (Gen.choose (0, 100))

Gen.choose (0, 100) |> Gen.map (fun x -> x * 2)

gen {
    let! xs = Gen.listOf (Gen.choose (0, 100))
    let! x = Gen.elements xs
    return (xs, x)
}

Quick Start (CsCheck)

csharp
using CsCheck;

[Fact]
public void Sort_Preserves_Length()
{
    Gen.Int.Array
       .Sample(arr => arr.OrderBy(x => x).Count() == arr.Length);
}

CsCheck Generators

csharp
Gen.Int                                // any int
Gen.Int[0, 100]                        // bounded
Gen.Double
Gen.String
Gen.Bool
Gen.Int.Array                          // int[]
Gen.Int.List                           // List<int>
Gen.Int.HashSet                        // HashSet<int>
Gen.Int.Select(x => x * 2)            // map
Gen.Select(Gen.String, Gen.Int, (name, age) => new User(name, age))
Gen.OneOf(Gen.Int.Select(x => (object)x), Gen.String.Select(x => (object)x))
Gen.Int.Where(x => x > 0)             // filter

Stateful Testing

FsCheck: No stateful testing support. Use CsCheck for stateful and parallel testing. fsharp-hedgehog: No stateful testing support.

CsCheck Stateful Testing

csharp
[Fact]
public void Store_Matches_Model()
{
    Gen.Int.List[0, 100].Sample(operations =>
    {
        var store = new MyStore();
        var model = new Dictionary<string, int>();
        // CsCheck uses operation sequences with Check.Sample
    });
}

CsCheck Parallel/Linearizability Testing

csharp
[Fact]
public void Store_Is_Linearizable()
{
    Check.SampleConcurrent(
        Gen.Operation<MyStore>(/* ... */),
        initialState: () => new MyStore()
    );
}

Check.SampleConcurrent runs operations sequentially then in parallel, checking against all possible linearizations. Shrinking works for parallel failures -- rare among PBT frameworks.

Quick Start (fsharp-hedgehog)

fsharp
open Hedgehog

let propReverse = property {
    let! xs = Gen.list (Range.linear 0 100) Gen.alpha
    return List.rev (List.rev xs) = xs
}

Property.check propReverse

fsharp-hedgehog Generators

fsharp
Gen.int (Range.linear 0 100)
Gen.string (Range.linear 0 50) Gen.alpha
Gen.bool
Gen.list (Range.linear 0 50) (Gen.int (Range.linear 0 100))
Gen.option (Gen.int (Range.linear 0 100))

Test Runner Integration

xml
<!-- FsCheck -->
<PackageReference Include="FsCheck.Xunit" Version="3.0.0" />
<!-- CsCheck -->
<PackageReference Include="CsCheck" Version="4.0.0" />
<!-- fsharp-hedgehog -->
<PackageReference Include="Hedgehog" Version="0.13.0" />
<PackageReference Include="Hedgehog.Xunit" Version="0.5.0" />
<!-- All work with xUnit, NUnit, MSTest (CsCheck/FsCheck) -->

Unique Features

FsCheck

  • F# + C# + VB.NET: Works across all .NET languages
  • Arb<T> type class: Type-based generation/shrinking, automatic derivation
  • v3 stable: Major rewrite with improved API and performance
  • Conditional properties: Prop.When for preconditions with automatic discard tracking
  • Model-based testing: Prop.ForAll with Command for lightweight model checking
  • Observable properties: Distribution analysis via Prop.Collect and Prop.Classify

CsCheck

  • Parallel linearizability testing: Check.SampleConcurrent -- rare capability
  • PCG-based shrinking: Parallelizable, fast, reproducible
  • Performance testing: Built-in Check.Faster for comparative benchmarks
  • Causal profiling: Built-in Check.CausalProfiling

fsharp-hedgehog

  • Computation expressions: gen { }, property { } -- most readable F# PBT syntax
  • Integrated shrinking: Automatic via rose trees
  • Hedgehog.Experimental: Auto-generators (AutoFixture-like)

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