CommandFlow 1.0.4

<p align="center"> <img src="assets/icon.svg" alt="CommandFlow" width="120" /> </p>

<h1 align="center">CommandFlow</h1>

<p align="center">A lightweight in-process messaging and CQRS library for .NET.</p>

<p align="center"> <a href="https://github.com/ViacheslavMelnichenko/command-flow/actions/workflows/ci.yml"><img src="https://github.com/ViacheslavMelnichenko/command-flow/actions/workflows/ci.yml/badge.svg" alt="CI"></a> <a href="https://www.nuget.org/packages/CommandFlow"><img src="https://img.shields.io/nuget/v/CommandFlow.svg" alt="NuGet"></a> <a href="https://github.com/ViacheslavMelnichenko/command-flow"><img src="https://img.shields.io/badge/coverage-100%25-brightgreen" alt="Coverage"></a> </p>

Overview

CommandFlow provides a clean, minimal implementation of the mediator pattern for .NET applications. It enables in-process messaging with support for commands, queries, notifications, and pipeline behaviors — all with first-class dependency injection support.

Goals

• Lightweight — minimal dependencies, small API surface
• Familiar — intuitive for developers experienced with CQRS/mediator patterns
• Production-ready — nullable-aware, cancellation-aware, deterministic behavior ordering
• Testable — easy to mock, easy to verify

Features

• ✅ Commands (void and with result)
• ✅ Queries (with result)
• ✅ Request/response pattern
• ✅ Notifications with multiple handlers
• ✅ Pipeline behaviors (middleware)
• ✅ Deterministic behavior execution order
• ✅ Microsoft.Extensions.DependencyInjection integration
• ✅ Assembly scanning for automatic handler registration
• ✅ Full CancellationToken flow
• ✅ Compatible with CSharpFunctionalExtensions Result types (no extra packages needed)

Installation

dotnet add package CommandFlow
dotnet add package CommandFlow.DependencyInjection

Quick Start

1. Define a request and handler

using CommandFlow;

public record GetUserQuery(int UserId) : IQuery<UserDto>;

public class GetUserQueryHandler : IRequestHandler<GetUserQuery, UserDto>
{
    public Task<UserDto> Handle(GetUserQuery request, CancellationToken cancellationToken)
    {
        var user = new UserDto(request.UserId, "Alice");
        return Task.FromResult(user);
    }
}

public record UserDto(int Id, string Name);

2. Register services

using Microsoft.Extensions.DependencyInjection;

var services = new ServiceCollection();
services.AddCommandFlow(typeof(GetUserQueryHandler).Assembly);

3. Send a request

var mediator = serviceProvider.GetRequiredService<IMediator>();
var user = await mediator.Send(new GetUserQuery(1));

Commands

Void command (no return value)

public record DeleteUserCommand(int UserId) : ICommand;

public class DeleteUserCommandHandler : ICommandHandler<DeleteUserCommand>
{
    public Task HandleCommand(DeleteUserCommand command, CancellationToken cancellationToken)
    {
        // Delete user logic
        return Task.CompletedTask;
    }
}

// Usage
await mediator.Send(new DeleteUserCommand(1));

Command with result

public record CreateUserCommand(string Name) : ICommand<int>;

public class CreateUserCommandHandler : IRequestHandler<CreateUserCommand, int>
{
    public Task<int> Handle(CreateUserCommand request, CancellationToken cancellationToken)
    {
        // Create user, return ID
        return Task.FromResult(42);
    }
}

// Usage
int userId = await mediator.Send(new CreateUserCommand("Alice"));

Queries

public record GetOrdersQuery(int UserId) : IQuery<List<OrderDto>>;

public class GetOrdersQueryHandler : IRequestHandler<GetOrdersQuery, List<OrderDto>>
{
    public Task<List<OrderDto>> Handle(GetOrdersQuery request, CancellationToken cancellationToken)
    {
        var orders = new List<OrderDto> { new(1, "Widget") };
        return Task.FromResult(orders);
    }
}

public record OrderDto(int Id, string ProductName);

Notifications

Notifications are dispatched to all registered handlers. Handlers execute sequentially in registration order. If a handler throws, subsequent handlers will not execute.

public record OrderPlacedEvent(int OrderId) : INotification;

public class SendEmailOnOrderPlaced : INotificationHandler<OrderPlacedEvent>
{
    public Task Handle(OrderPlacedEvent notification, CancellationToken cancellationToken)
    {
        // Send confirmation email
        return Task.CompletedTask;
    }
}

public class UpdateInventoryOnOrderPlaced : INotificationHandler<OrderPlacedEvent>
{
    public Task Handle(OrderPlacedEvent notification, CancellationToken cancellationToken)
    {
        // Update inventory
        return Task.CompletedTask;
    }
}

// Usage
await mediator.Publish(new OrderPlacedEvent(42));

Notification execution strategy

Notifications use sequential execution by design:

• Handlers are invoked one at a time in the order they were registered
• If a handler throws an exception, the remaining handlers are not executed
• The exception propagates to the caller
• This is the simplest and most predictable strategy

Pipeline Behaviors

Pipeline behaviors wrap request handling, enabling cross-cutting concerns like validation, logging, caching, and performance monitoring.

Behaviors execute in registration order: the first registered behavior is the outermost wrapper.

public class LoggingBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly ILogger<LoggingBehavior<TRequest, TResponse>> _logger;

    public LoggingBehavior(ILogger<LoggingBehavior<TRequest, TResponse>> logger)
    {
        _logger = logger;
    }

    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken)
    {
        _logger.LogInformation("Handling {RequestType}", typeof(TRequest).Name);
        var response = await next();
        _logger.LogInformation("Handled {RequestType}", typeof(TRequest).Name);
        return response;
    }
}

Behavior execution order

Request → Behavior1 → Behavior2 → Handler → Behavior2 → Behavior1 → Response

Register behaviors in the order you want them to wrap:

services.AddCommandFlow(typeof(MyHandler).Assembly)
    .AddBehavior(typeof(LoggingBehavior<,>))        // outermost
    .AddBehavior(typeof(ValidationBehavior<,>));     // inner

Short-circuiting

A behavior can skip calling next() to short-circuit the pipeline:

public class CacheBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken)
    {
        // Check cache, return early if found
        // Otherwise call next() and cache the result
        return await next();
    }
}

Result-Based Responses (CSharpFunctionalExtensions)

CommandFlow is fully compatible with CSharpFunctionalExtensions Result types. No special integration package or abstractions are needed — Result, Result<T>, UnitResult<TError>, and Result<TValue, TError> are simply used as TResponse.

This works because CommandFlow's generics are open: ICommand<TResult>, IQuery<TResult>, IRequestHandler<TRequest, TResponse>, and IPipelineBehavior<TRequest, TResponse> accept any response type.

When to use Result vs exceptions

Rule of thumb: if the caller is expected to handle the failure as part of normal flow, return a Result. If the failure is unexpected and indicates a bug or infrastructure issue, throw.

Command returning Result

using CSharpFunctionalExtensions;

public record PlaceOrderCommand(string ProductName, int Quantity) : ICommand<Result>;

public class PlaceOrderHandler : IRequestHandler<PlaceOrderCommand, Result>
{
    public Task<Result> Handle(PlaceOrderCommand request, CancellationToken cancellationToken)
    {
        if (request.Quantity <= 0)
            return Task.FromResult(Result.Failure("Quantity must be positive"));

        return Task.FromResult(Result.Success());
    }
}

var result = await mediator.Send(new PlaceOrderCommand("Widget", 3));
if (result.IsFailure)
    Console.WriteLine(result.Error);

Command returning Result<T>

public record CreateOrderCommand(string Name, int Qty) : ICommand<Result<int>>;

public class CreateOrderHandler : IRequestHandler<CreateOrderCommand, Result<int>>
{
    public Task<Result<int>> Handle(CreateOrderCommand request, CancellationToken cancellationToken)
    {
        if (request.Qty <= 0)
            return Task.FromResult(Result.Failure<int>("Quantity must be positive"));

        return Task.FromResult(Result.Success(42));
    }
}

Query returning Result<T>

public record FindOrderQuery(int OrderId) : IQuery<Result<OrderDto>>;

public class FindOrderHandler : IRequestHandler<FindOrderQuery, Result<OrderDto>>
{
    public Task<Result<OrderDto>> Handle(FindOrderQuery request, CancellationToken cancellationToken)
    {
        if (request.OrderId <= 0)
            return Task.FromResult(Result.Failure<OrderDto>("Invalid order ID"));

        return Task.FromResult(Result.Success(new OrderDto(request.OrderId, "Widget", 5)));
    }
}

Typed errors with Result<TValue, TError>

public record TransferError(string Code, string Message);
public record TransferFundsCommand(decimal Amount) : ICommand<Result<decimal, TransferError>>;

public class TransferHandler : IRequestHandler<TransferFundsCommand, Result<decimal, TransferError>>
{
    public Task<Result<decimal, TransferError>> Handle(
        TransferFundsCommand request, CancellationToken cancellationToken)
    {
        if (request.Amount <= 0)
            return Task.FromResult(
                Result.Failure<decimal, TransferError>(
                    new TransferError("INVALID", "Amount must be positive")));

        return Task.FromResult(Result.Success<decimal, TransferError>(request.Amount));
    }
}

UnitResult<TError>

public record ArchiveOrderCommand(int OrderId) : ICommand<UnitResult<string>>;

public class ArchiveOrderHandler : IRequestHandler<ArchiveOrderCommand, UnitResult<string>>
{
    public Task<UnitResult<string>> Handle(ArchiveOrderCommand request, CancellationToken cancellationToken)
    {
        if (request.OrderId <= 0)
            return Task.FromResult(UnitResult.Failure("Invalid order ID"));

        return Task.FromResult(UnitResult.Success<string>());
    }
}

Pipeline behaviors work transparently

Generic behaviors compose with Result-based handlers without special handling. A Result.Failure(...) is a normal return value, not an exception — it flows through the pipeline like any other response:

services.AddCommandFlow(typeof(PlaceOrderHandler).Assembly)
    .AddBehavior(typeof(LoggingBehavior<,>));

Dependency Injection Setup

Basic registration

services.AddCommandFlow(typeof(MyHandler).Assembly);

With pipeline behaviors

services.AddCommandFlow(typeof(MyHandler).Assembly)
    .AddBehavior(typeof(LoggingBehavior<,>))
    .AddBehavior(typeof(ValidationBehavior<,>));

Multiple assemblies

services.AddCommandFlow(
    typeof(MyHandler).Assembly,
    typeof(AnotherHandler).Assembly
);

Additional assembly scanning

services.AddCommandFlow(typeof(MyHandler).Assembly)
    .AddHandlersFromAssemblies(typeof(PluginHandler).Assembly);

What gets registered

Examples

See the /docs folder for complete examples:

• Command
• Command with result
• Query
• Notification
• Multiple notification handlers
• Pipeline behavior
• DI registration
• Assembly scanning
• Cancellation
• Result-based responses
• Testing

Testing

CommandFlow is designed to be easy to test. You can:

1. Test handlers directly — they're simple classes with a Handle method
2. Mock IMediator — verify that the correct requests are sent
3. Integration test with real DI — build a real service provider for end-to-end tests

// Direct handler test
var handler = new GetUserQueryHandler();
var result = await handler.Handle(new GetUserQuery(1), CancellationToken.None);
Assert.Equal("Alice", result.Name);

// Mock mediator
var mediator = new Mock<IMediator>();
mediator.Setup(m => m.Send(It.IsAny<GetUserQuery>(), It.IsAny<CancellationToken>()))
    .ReturnsAsync(new UserDto(1, "Alice"));

Packaging

The library is split into two NuGet packages:

• CommandFlow — core abstractions and mediator implementation (zero external dependencies)
• CommandFlow.DependencyInjection — DI registration extensions (depends on Microsoft.Extensions.DependencyInjection.Abstractions)

Benchmarks

Run benchmarks with:

dotnet run --project benchmarks/CommandFlow.Benchmarks -c Release -- --filter "*"

Benchmarks measure:

• Request dispatch overhead
• Pipeline behavior overhead
• Notification publishing with multiple handlers

Architecture

IMediator : ISender, IPublisher
    │
    ├── Send<TResponse>(IRequest<TResponse>)
    │       │
    │       ├── IPipelineBehavior<TRequest, TResponse> (0..N)
    │       │       │
    │       │       └── IRequestHandler<TRequest, TResponse>
    │       │
    │       ├── ICommand → IRequest<Unit>
    │       ├── ICommand<TResult> → IRequest<TResult>
    │       └── IQuery<TResult> → IRequest<TResult>
    │
    └── Publish<TNotification>(TNotification)
            │
            └── INotificationHandler<TNotification> (0..N, sequential)

License

MIT