DotnetHandler

A lightweight, modular request handling package for .NET.

A lightweight, modular .NET Standard library that provides request/response handlers, a pipeline, an event bus, built-in validation, and a fluent registration API — all with zero forced patterns.

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Features

Feature	Description
Dispatcher	Unified IDispatcher with Send (1:1) and Publish (1:N)
Handlers	Strongly-typed IRequestHandler<TRequest, TResponse>
Validation	Automatic, opt-in per-handler via IValidationHandler<TRequest>
Pipeline	Ordered middleware via IPipelineBehavior<TRequest, TResponse>
Events	Fire-and-forget pub/sub via IEvent / IEventListener<TEvent>
Cache Behavior	Opt-in response caching via ICacheableRequest<TResponse>
Permission Behavior	Opt-in authorization via IAuthorizedRequest + IPermissionContext
Idempotency Behavior	Duplicate-request prevention via IIdempotentRequest
Fluent API	Explicit registration with AddDotnetHandler(...)
Source Generator	Zero-reflection registration via UseGeneratedHandlers()
Assembly scanning	Reflection-based auto-discovery via FromAssembly(...) (legacy)
Modular	Use only the modules you need

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Installation

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Quick Start

1. Define a request

public record CreateUserCommand(string Name, string Email) : IRequest<User>;

2. Implement a handler (with optional built-in validation)

public class CreateUserHandler
    : IRequestHandler<CreateUserCommand, User>,
      IValidationHandler<CreateUserCommand>   // optional
{
    public Task<ValidationResult> ValidateAsync(CreateUserCommand request)
    {
        if (string.IsNullOrWhiteSpace(request.Name))
            return Task.FromResult(ValidationResult.Failure("Name is required."));

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

    public Task<User> HandleAsync(CreateUserCommand request) =>
        Task.FromResult(new User(Guid.NewGuid(), request.Name, request.Email));
}

3. Register

Recommended — source generator (zero reflection):

// All IRequestHandler<,> and IEventListener<> in this assembly are registered automatically.
builder.Services.AddDotnetHandler(app =>
{
    app.UseGeneratedHandlers();

    // Pipeline behaviors are always explicit (open-generic supported)
    app.Pipeline(p => p.Use(typeof(LoggingBehavior<,>)));
});

Alternative — fluent (explicit, no generator needed):

builder.Services.AddDotnetHandler(app =>
{
    app.Handlers(h =>
        h.Register<CreateUserCommand, User>().HandledBy<CreateUserHandler>());

    app.Events(e =>
        e.Register<UserCreatedEvent>().Subscribe<SendWelcomeEmailListener>());

    app.Pipeline(p => p.Use(typeof(LoggingBehavior<,>)));
});

4. Dispatch

app.MapPost("/users", async (CreateUserCommand cmd, IDispatcher dispatcher) =>
{
    try
    {
        var user = await dispatcher.Send(cmd);
        return Results.Ok(user);
    }
    catch (ValidationException ex)
    {
        return Results.BadRequest(new { errors = ex.Errors });
    }
});

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Core Abstractions

Requests & Handlers

public interface IRequest<TResponse> { }

public interface IRequestHandler<TRequest, TResponse>
{
    Task<TResponse> HandleAsync(TRequest request);
}

Events

public interface IEvent { }

public interface IEventListener<TEvent>
{
    Task HandleAsync(TEvent @event);
}

Pipeline

public interface IPipelineBehavior<TRequest, TResponse>
{
    Task<TResponse> HandleAsync(TRequest request, Func<Task<TResponse>> next);
}

Validation

public interface IValidationHandler<TRequest>
{
    Task<ValidationResult> ValidateAsync(TRequest request);
}

Validation runs automatically before the handler and pipeline when the handler also implements IValidationHandler<TRequest>. A ValidationException is thrown on failure — no manual wiring needed.

Behavior contracts

// Opt-in response caching
public interface ICacheableRequest<TResponse>
{
    string CacheKey { get; }
    TimeSpan? CacheDuration { get; }
}

// Opt-in authorization
public interface IAuthorizedRequest
{
    IEnumerable<string> RequiredPermissions { get; }
}

// Current-user permission check (implement in your application)
public interface IPermissionContext
{
    bool HasPermission(string permission);
}

// Opt-in idempotency
public interface IIdempotentRequest
{
    string IdempotencyKey { get; }
}

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Dispatcher Behaviour

Operation	Validation	Pipeline	Listeners	Throws if none
Send	✅ (if handler implements IValidationHandler)	✅	—	✅
Publish	❌	❌	All matching	❌

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Source Generator

The source generator ships with the package. It runs at compile time and emits a UseGeneratedHandlers() extension method for your assembly — no reflection at runtime.

What it discovers automatically:

Type	Registered via
IRequestHandler<TRequest, TResponse>	app.Handlers(...)
IEventListener<TEvent>	app.Events(...)

What must remain explicit:

Type	Why
IPipelineBehavior<,>	Order matters — always declare in app.Pipeline(...)
FluentValidation validators	External dependency, not a DotnetHandler abstraction

Pipeline behaviors and external validators are not auto-registered by design.

Usage

builder.Services.AddDotnetHandler(app =>
{
    app.UseGeneratedHandlers();
    app.Pipeline(p => p.Use(typeof(LoggingBehavior<,>)));
});

The generated code is emitted to obj/{config}/{tfm}/generated/DotnetHandler.SourceGenerators/.../*.g.cs and is visible in your IDE.

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Assembly Scanning (legacy)

Runtime reflection fallback — useful for plugin scenarios or when source generators are unavailable:

builder.Services.AddDotnetHandler(app =>
    app.FromAssembly(typeof(Program).Assembly));

Auto-registers: IRequestHandler<,>, IEventListener<>
Does NOT auto-register: pipeline behaviors (always explicit)

Pipeline Behaviors

Closed (single request type)

app.Pipeline(p => p.Use<MyBehavior>());

Open generic (all requests)

app.Pipeline(p => p.Use(typeof(LoggingBehavior<,>)));

Behaviors execute in registration order (first registered = outermost wrapper).

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Cache Behavior

Mark a query as cacheable by implementing ICacheableRequest<TResponse>:

public record GetUsersQuery : IRequest<List<User>>, ICacheableRequest<List<User>>
{
    public string CacheKey => "users:all";
    public TimeSpan? CacheDuration => TimeSpan.FromMinutes(1); // null = 5 min default
}

Implement the behavior using IMemoryCache (or any cache abstraction):

public class CacheBehavior<TRequest, TResponse>(IMemoryCache cache)
    : IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> HandleAsync(TRequest request, Func<Task<TResponse>> next)
    {
        if (request is not ICacheableRequest<TResponse> cacheable)
            return await next();

        if (cache.TryGetValue(cacheable.CacheKey, out TResponse? cached))
            return cached!;

        var result = await next();
        cache.Set(cacheable.CacheKey, result, cacheable.CacheDuration ?? TimeSpan.FromMinutes(5));
        return result;
    }
}

Register:

builder.Services.AddMemoryCache();
app.Pipeline(p => p.Use(typeof(CacheBehavior<,>)));

Non-cacheable requests pass through transparently.

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Permission Behavior

Implement IPermissionContext to provide the current user's permissions:

public interface IPermissionContext
{
    bool HasPermission(string permission);
}

Mark a command as requiring authorization by implementing IAuthorizedRequest:

public record DeleteUserCommand(Guid Id) : IRequest<bool>, IAuthorizedRequest
{
    public IEnumerable<string> RequiredPermissions => ["users:delete"];
}

Implement the behavior:

public class PermissionBehavior<TRequest, TResponse>(IPermissionContext context)
    : IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> HandleAsync(TRequest request, Func<Task<TResponse>> next)
    {
        if (request is not IAuthorizedRequest authorized)
            return await next();

        foreach (var permission in authorized.RequiredPermissions)
        {
            if (!context.HasPermission(permission))
                throw new UnauthorizedException(permission);
        }

        return await next();
    }
}

UnauthorizedException exposes the Permission property. Catch it at the endpoint to return 403:

app.MapDelete("/users/{id:guid}", async (Guid id, IDispatcher dispatcher) =>
{
    try { ... }
    catch (UnauthorizedException ex)
    {
        return Results.Problem(ex.Message, statusCode: 403);
    }
});

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Idempotency Behavior

Mark a command as idempotent by implementing IIdempotentRequest:

public record CreateUserCommand(string Name, string Email, string IdempotencyKey = "")
    : IRequest<UserResponse>, IIdempotentRequest;

The client sends the key in the Idempotency-Key header; the endpoint injects it into the command before dispatching. Repeated requests with the same key return the stored result without re-executing the handler.

Implement the behavior:

public class IdempotencyBehavior<TRequest, TResponse>(IMemoryCache cache)
    : IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> HandleAsync(TRequest request, Func<Task<TResponse>> next)
    {
        if (request is not IIdempotentRequest idempotent || string.IsNullOrWhiteSpace(idempotent.IdempotencyKey))
            return await next();

        var key = $"idempotency:{idempotent.IdempotencyKey}";
        if (cache.TryGetValue(key, out TResponse? stored))
            return stored!;

        var result = await next();
        cache.Set(key, result, TimeSpan.FromHours(24));
        return result;
    }
}

Requests with a blank IdempotencyKey bypass the behavior and always execute.

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Validation Details

ValidationResult is a simple value object:

ValidationResult.Success();
ValidationResult.Failure("Error 1", "Error 2");

ValidationException exposes IReadOnlyCollection<string> Errors.

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Project Structure

DotnetHandler.sln
├── src/
│   ├── DotnetHandler/                  # Core library (netstandard2.1)
│   │   ├── Abstractions/               # Interfaces
│   │   ├── Core/                       # Dispatcher implementation
│   │   ├── Validation/                 # ValidationResult, ValidationException
│   │   ├── Registration/               # Fluent API, ApplicationBuilder
│   │   └── Internal/                   # Assembly scanner (legacy)
│   └── DotnetHandler.SourceGenerators/ # Roslyn source generator (netstandard2.0)
├── tests/
│   ├── DotnetHandler.Tests/            # Core unit tests (net10.0)
│   └── DotnetHandler.Sample.Tests/     # Integration tests for sample (net10.0)
└── samples/
    └── DotnetHandler.Sample/           # Minimal API sample (net10.0)

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Design Goals

• As simple as Coravel
• As structured as MediatR
• Built-in validation without a framework dependency
• No enforced response pattern (return whatever TResponse you want)
• Explicit and predictable — no magic beyond optional assembly scanning