KnockOff 10.33.0

KnockOff

No more Arg.Any<>(). No more It.IsAny<>(). Just write C#.

The Difference

NSubstitute:

var repo = Substitute.For<IUserRepo>();
repo.GetUser(Arg.Is<int>(id => id > 0)).Returns(x => new User { Id = x.Arg<int>() });

KnockOff:

var stub = new UserRepoStub();
stub.GetUser.OnCall((id) => id > 0 ? new User { Id = id } : null);

No Arg.Is<>(). No x.Arg<int>(). The parameter is just id.

Unique Feature: Source Delegation

Delegate to a real implementation, override only what you need:

var realRepo = new SqlUserRepository(connectionString);
var stub = new UserRepoStub();

stub.Source(realRepo);  // ALL methods delegate to real implementation

// Override just the method you're testing
stub.GetUser.OnCall((id) => new User { Id = id, Name = "Test User" });

IUserRepo repo = stub;
repo.Save(user);     // Calls real SqlUserRepository.Save()
repo.GetUser(1);     // Returns test data

No other mocking framework has this. Perfect for integration tests, decorator patterns, and partial mocking without complexity.

Side-by-Side Comparisons

Methods

Argument Matching

// NSubstitute - Arg.Is<T> per parameter (permanent matchers)
calc.Add(Arg.Is<int>(a => a > 0), Arg.Any<int>()).Returns(100);

// KnockOff - OnCall with conditional (permanent, matches all calls)
stub.Add.OnCall((a, b) => a > 0 ? 100 : 0);

// KnockOff - When() for sequential matching (first match returns 100, then falls through)
stub.Add.When((a, b) => a > 0).Returns(100).ThenCall((a, b) => a + b);

// Multiple specific values
calc.Add(1, 2).Returns(100);
calc.Add(3, 4).Returns(200);

stub.Add.When(1, 2).Returns(100);
stub.Add.When(3, 4).Returns(200);

Note: NSubstitute's matchers are permanent—they match all qualifying calls. KnockOff's When() is sequential—matchers are consumed in order. Use OnCall() with conditionals for permanent matching behavior.

Argument Capture

// NSubstitute - requires Arg.Do in setup
int capturedA = 0, capturedB = 0;
calc.Add(Arg.Do<int>(x => capturedA = x), Arg.Do<int>(x => capturedB = x));
calc.Add(1, 2);

// KnockOff - built-in, no pre-setup
var tracking = stub.Add.OnCall((a, b) => a + b);
calc.Add(1, 2);
var (a, b) = tracking.LastArgs;  // Named tuple: a = 1, b = 2

Properties

Events

Delegates

Indexers

Feature Parity

KnockOff covers the features NSubstitute users expect:

What KnockOff Does Better

Quick Start

Install

dotnet add package KnockOff

Create a Stub

public interface IQuickStartRepo
{
    User? GetUser(int id);
}

[KnockOff]
public partial class QuickStartRepoStub : IQuickStartRepo { }

public class QuickStartCreateStubTests
{
    [Fact]
    public void CreateStub_IsReady()
    {
        var stub = new QuickStartRepoStub();

        IQuickStartRepo repository = stub;
        Assert.NotNull(repository);
    }
}

Configure and Verify

[Fact]
public void ConfigureStub_WithOnCall()
{
    var stub = new QuickStartRepoStub();

    stub.GetUser.OnCall((id) => new User { Id = id, Name = "Test User" });

    IQuickStartRepo repository = stub;
    var user = repository.GetUser(42);

    Assert.NotNull(user);
    Assert.Equal(42, user.Id);
    Assert.Equal("Test User", user.Name);
}

[Fact]
public void VerifyCalls_WithVerifiable()
{
    var stub = new QuickStartRepoStub();
    stub.GetUser.OnCall((id) => new User { Id = id, Name = "Test" }).Verifiable();

    IQuickStartRepo repository = stub;

    var user = repository.GetUser(42);

    // Verify() checks all members marked with .Verifiable()
    stub.Verify();
}

Three Stub Patterns

Standalone - Reusable across your project:

[KnockOff]
public partial class UserRepoStub : IUserRepo { }

Inline Interface - Test-local stubs:

[KnockOff<IUserRepo>]
public partial class MyTests
{
    [Fact]
    public void Test()
    {
        var stub = new Stubs.IUserRepo();
    }
}

Inline Class - Stub virtual members:

[KnockOff<MyService>]
public partial class MyTests
{
    [Fact]
    public void Test()
    {
        var stub = new Stubs.MyService();
        IMyService service = stub.Object;
    }
}

Documentation

• Getting Started - Installation and first stub
• Stub Patterns - Standalone, inline interface, inline class
• Interceptor API - Complete OnCall, OnGet, OnSet reference
• Source Delegation - Delegate to real implementations
• Migration from Moq - Step-by-step migration guide
• Migration from NSubstitute - Comparison and migration guide

License

MIT License. See LICENSE for details.

Contributing

Contributions welcome! See CONTRIBUTING.md for guidelines.

• Issues: GitHub Issues
• Pull Requests: Bug fixes, features, documentation
• Discussions: GitHub Discussions