MediatR Handlers

Core Principles

• One handler per request — single responsibility
• Commands use IRequest<TResponse> for operations that change state
• Queries use IRequest<TResponse> for read-only data retrieval
• Notifications use INotification for one-to-many event dispatch
• Register all handlers from assembly with RegisterServicesFromAssembly

Patterns

Command Handler

// Command record — immutable, represents intent
public sealed record CreateOrderCommand(
    string CustomerName,
    List<CreateOrderCommand.ItemDto> Items) : IRequest<Result<Guid>>
{
    public sealed record ItemDto(Guid ProductId, int Quantity);
}

// Handler — internal, one per command
internal sealed class CreateOrderCommandHandler(
    IOrderRepository repository,
    IUnitOfWork unitOfWork,
    ILogger<CreateOrderCommandHandler> logger)
    : IRequestHandler<CreateOrderCommand, Result<Guid>>
{
    public async Task<Result<Guid>> Handle(
        CreateOrderCommand request, CancellationToken ct)
    {
        var order = Order.Create(request.CustomerName);

        foreach (var item in request.Items)
            order.AddItem(item.ProductId, item.Quantity);

        repository.Add(order);
        await unitOfWork.SaveChangesAsync(ct);

        logger.LogInformation(
            "Created order {OrderId} for {Customer}",
            order.Id, request.CustomerName);

        return Result<Guid>.Success(order.Id);
    }
}

Query Handler

// Query record
public sealed record GetOrderQuery(Guid OrderId)
    : IRequest<OrderResponse?>;

// Handler with DbContext (read-only, no repository needed)
internal sealed class GetOrderQueryHandler(AppDbContext db)
    : IRequestHandler<GetOrderQuery, OrderResponse?>
{
    public async Task<OrderResponse?> Handle(
        GetOrderQuery request, CancellationToken ct)
    {
        return await db.Orders
            .AsNoTracking()
            .Where(o => o.Id == request.OrderId)
            .Select(o => new OrderResponse(
                o.Id,
                o.CustomerName,
                o.Total,
                o.Status.ToString(),
                o.CreatedAt,
                o.Items.Select(i => new OrderItemResponse(
                    i.ProductId, i.ProductName,
                    i.Quantity, i.UnitPrice)).ToList()))
            .FirstOrDefaultAsync(ct);
    }
}

Void Command (No Return Value)

// IRequest with no generic parameter = Unit return
public sealed record DeleteOrderCommand(Guid OrderId) : IRequest;

internal sealed class DeleteOrderCommandHandler(
    IOrderRepository repository,
    IUnitOfWork unitOfWork)
    : IRequestHandler<DeleteOrderCommand>
{
    public async Task Handle(
        DeleteOrderCommand request, CancellationToken ct)
    {
        var order = await repository.FindAsync(request.OrderId, ct)
            ?? throw new NotFoundException("Order", request.OrderId);

        repository.Remove(order);
        await unitOfWork.SaveChangesAsync(ct);
    }
}

Notification Handler

// Notification — dispatched to multiple handlers
public sealed record OrderCreatedNotification(
    Guid OrderId,
    string CustomerName) : INotification;

// Handler 1: send email
internal sealed class SendOrderConfirmationEmail(
    IEmailService emailService)
    : INotificationHandler<OrderCreatedNotification>
{
    public async Task Handle(
        OrderCreatedNotification notification,
        CancellationToken ct)
    {
        await emailService.SendOrderConfirmationAsync(
            notification.OrderId, notification.CustomerName, ct);
    }
}

// Handler 2: update analytics
internal sealed class UpdateOrderAnalytics(AppDbContext db)
    : INotificationHandler<OrderCreatedNotification>
{
    public async Task Handle(
        OrderCreatedNotification notification,
        CancellationToken ct)
    {
        var stats = await db.DashboardStats.SingleAsync(ct);
        stats.IncrementOrderCount();
        await db.SaveChangesAsync(ct);
    }
}

Registration

// Program.cs — register all handlers from assembly
builder.Services.AddMediatR(cfg =>
{
    cfg.RegisterServicesFromAssembly(typeof(Program).Assembly);

    // Or from multiple assemblies
    cfg.RegisterServicesFromAssemblies(
        typeof(CreateOrderCommand).Assembly,
        typeof(OrderCreatedNotification).Assembly);
});

Dispatching from Endpoints

// ISender for commands and queries
app.MapPost("/orders", async (
    CreateOrderCommand cmd,
    ISender sender,
    CancellationToken ct) =>
{
    var result = await sender.Send(cmd, ct);
    return result.Match(
        id => Results.Created($"/orders/{id}", new { id }),
        error => Results.BadRequest(error.ToProblemDetails()));
});

// IPublisher for notifications
app.MapPost("/orders/{id}/submit", async (
    Guid id, ISender sender, IPublisher publisher,
    CancellationToken ct) =>
{
    await sender.Send(new SubmitOrderCommand(id), ct);
    await publisher.Publish(
        new OrderSubmittedNotification(id), ct);
    return Results.NoContent();
});

Naming Conventions

Type	Pattern	Example
Command	{Verb}{Noun}Command	CreateOrderCommand
Query	Get{Noun}Query / List{Noun}Query	GetOrderQuery
Handler	{Command/Query}Handler	CreateOrderCommandHandler
Notification	{Noun}{PastVerb}Notification	OrderCreatedNotification
Validator	{Command}Validator	CreateOrderCommandValidator

Anti-Patterns

• Multiple handlers for one request (MediatR allows only one per IRequest)
• Business logic in notification handlers (keep them for side effects only)
• Public handler classes (use internal sealed)
• Not propagating CancellationToken through the handler chain
• Returning full entities from handlers (return DTOs or IDs)

Detect Existing Patterns

1. Search for IRequest< and IRequestHandler< implementations
2. Look for INotification and INotificationHandler< implementations
3. Check for MediatR package reference in .csproj
4. Look for ISender or IMediator injection
5. Check for RegisterServicesFromAssembly calls

Adding to Existing Project

1. Install MediatR — dotnet add package MediatR
2. Register with AddMediatR in Program.cs
3. Create command/query records following naming conventions
4. Create handler classes — one per command/query, internal sealed
5. Dispatch from endpoints using ISender.Send
6. Add notifications for cross-cutting side effects

References

• https://github.com/jbogard/MediatR
• https://github.com/jbogard/MediatR/wiki