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MediatR is a low-ambition library trying to solve a simple problem — decoupling the in-process sending of messages from handling messages. Cross-platform, supporting netstandard2.0.

Setup

Install the package via NuGet first: Install-Package MediatR

MediatR directly references Microsoft.Extensions.DependencyInjection.Abstractions leveraging IServiceProvider. Typical usage is to use IServiceCollection directly:

services.AddMediatR(cfg => {
    cfg.RegisterServicesFromAssembly(typeof(Program).Assembly);
});

This method registers the known MediatR types:

IMediator as transient
ISender as transient
IPublisher as transient

For each assembly registered, the AddMediatR method will scan those assemblies for MediatR types (excluding behaviors):

IRequestHandler<,> concrete implementations as transient
IRequestHandler<> concrete implementations as transient
INotificationHandler<> concrete implementations as transient
IStreamRequestHandler<> concrete implementations as transient
IRequestExceptionHandler<,,> concrete implementations as transient
IRequestExceptionAction<,>) concrete implementations as transient

Behaviors and pre/post processors must be registered explicitly through the AddXyz methods.

Basics

MediatR has two kinds of messages it dispatches:

Request/response messages, dispatched to a single handler
Notification messages, dispatched to multiple handlers

Request/response

The request/response interface handles both command and query scenarios. First, create a message:

public class Ping : IRequest<string> { }

Next, create a handler:

public class PingHandler : IRequestHandler<Ping, string>
{
    public Task<string> Handle(Ping request, CancellationToken cancellationToken)
    {
        return Task.FromResult("Pong");
    }
}

Finally, send a message through the mediator:

var response = await mediator.Send(new Ping());
Debug.WriteLine(response); // "Pong"

In the case your message does not require a response, implement the non-generic IRequest interface and subsequent handler:

public class OneWay : IRequest { }
public class OneWayHandler : IRequestHandler<OneWay>
{
    public Task Handle(OneWay request, CancellationToken cancellationToken)
    {
        // do work
        return Task.CompletedTask;
    }
}

Request types

There are two flavors of requests in MediatR - ones that return a value, and ones that do not:

IRequest<TResponse> - the request returns a value
IRequest - the request does not return a value

Each request type has its own handler interface:

IRequestHandler<TRequest, TResponse> - implement this and return Task<TResponse>

Then for requests without return values:

IRequestHandler<TRequest> - implement this and you will return Task.

Streams and AsyncEnumerables

To create a stream from a request, first implement the stream request and its response:

IStreamRequest<TResponse>

public sealed class CounterStreamRequest : IStreamRequest<int> { }

Stream request handlers are separate from the normal IRequestHandler and require implementing:

IStreamRequestHandler<TRequest, TResponse>

public sealed class CounterStreamHandler : IStreamRequestHandler<CounterStreamRequest, int>
{
    public async IAsyncEnumerable<int> Handle(CounterStreamRequest request, [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        int count = 0;
        while (!cancellationToken.IsCancellationRequested)
        {
            await Task.Delay(500, cancellationToken);
            yield return count;
            count++;
        }
    }
}

Unlike normal request handlers that return a single TResponse, a stream handler returns an IAsyncEnumerable<TResponse>:

IAsyncEnumerable<TResponse> Handle(TRequest request, CancellationToken cancellationToken);

To create a stream request handler, create a class that implements IStreamRequestHandler<TRequest, TResponse> and implement the above Handle method.

Finally, send a stream message through the mediator:

    CancellationTokenSource cts = new();
    int count = 10;
    await foreach (var item in mediator.CreateStream<int>(new CounterStreamRequest(), cts.Token))
    {
        count--;
        if (count == 0)
        {
            cts.Cancel();
        }
        Debug.WriteLine(item);
    }

Notifications

For notifications, first create your notification message:

public class Ping : INotification { }

Next, create zero or more handlers for your notification:

public class Pong1 : INotificationHandler<Ping>
{
    public Task Handle(Ping notification, CancellationToken cancellationToken)
    {
        Debug.WriteLine("Pong 1");
        return Task.CompletedTask;
    }
}

public class Pong2 : INotificationHandler<Ping>
{
    public Task Handle(Ping notification, CancellationToken cancellationToken)
    {
        Debug.WriteLine("Pong 2");
        return Task.CompletedTask;
    }
}

Finally, publish your message via the mediator:

await mediator.Publish(new Ping());

Custom notification publishers

MediatR (starting with version 12) supports custom publish strategies:

public interface INotificationPublisher
{
    Task Publish(IEnumerable<NotificationHandlerExecutor> handlerExecutors, INotification notification,
        CancellationToken cancellationToken);
}

Custom publish strategies are injected into the Mediator class, and are registered with AddMediatR:

services.AddMediatR(cfg => {
    cfg.RegisterServicesFromAssemblyContaining<Program>();
    cfg.NotificationPublisher = new MyCustomPublisher(); // this will be singleton
    cfg.NotificationPublisherType = typeof(MyCustomPublisher); // this will be the ServiceLifetime
});

There are two built-in notification publishers:

ForeachAwaitPublisher - the default, existing implementation
TaskWhenAllPublisher - awaits Task.WhenAll for all notification handler tasks. Does NOT use Task.Run

Custom notification publishers can also use the handler instance to do custom logic like ordering, skipping handlers, etc. The handler instance is on the NotificationHandlerExecutor class, as well as the delegate to call the handler instance itself.

Polymorphic dispatch

Handler interfaces are contravariant:

public interface IRequestHandler<in TRequest, TResponse> where TRequest : IRequest<TResponse>
{
    Task<TResponse> Handle(TRequest message, CancellationToken cancellationToken);
}

public interface INotificationHandler<in TNotification>
{
    Task Handle(TNotification notification, CancellationToken cancellationToken);
}

Containers that support generic variance will dispatch accordingly. For example, you can have an INotificationHandler<INotification> to handle all notifications.

Async

Send/publish are asynchronous from the IMediator side, with corresponding synchronous and asynchronous-based interfaces/base classes for requests/responses/notification handlers.

Your handlers can use the async/await keywords as long as the work is awaitable:

public class PingHandler : IRequestHandler<Ping, Pong>
{
    public async Task<Pong> Handle(Ping request, CancellationToken cancellationToken)
    {
        await DoPong(); // Whatever DoPong does
    }
}

You will also need to register these handlers with the IoC container of your choice, similar to the synchronous handlers shown above.

Exceptions handling

Exception handler pipeline step

Exception handler implemented by using IPipelineBehavior concept. It requires to add the RequestExceptionProcessorBehavior to the request execution Pipeline. This behavior is not registered unless AddMediatR finds request exception behaviors.

namespace MediatR.Pipeline;

/// <summary>
/// Behavior for executing all <see cref="IRequestExceptionHandler{TRequest,TResponse,TException}"/> instances
///     after an exception is thrown by the following pipeline steps
/// </summary>
/// <typeparam name="TRequest">Request type</typeparam>
/// <typeparam name="TResponse">Response type</typeparam>
public class RequestExceptionProcessorBehavior<TRequest, TResponse, TException> : IRequestExceptionHandler<TRequest, TResponse, TException>
    where TRequest : notnull
{

Exception action pipeline step

Exception action implemented by using IPipelineBehavior concept. It requires to add the RequestExceptionActionProcessorBehavior to the request execution Pipeline. If place RequestExceptionActionProcessorBehavior before RequestExceptionProcessorBehavior, actions will be called only for unhandled exceptions.

namespace MediatR.Pipeline;

/// <summary>
/// Behavior for executing all <see cref="IRequestExceptionAction{TRequest,TException}"/> instances
///     after an exception is thrown by the following pipeline steps
/// </summary>
/// <typeparam name="TRequest">Request type</typeparam>
/// <typeparam name="TResponse">Response type</typeparam>
public class RequestExceptionActionProcessorBehavior<TRequest, TResponse> : IRequestExceptionAction<TRequest, TException>
    where TRequest : notnull
{

Handlers and actions priority execution

All available handlers/actions will be sorted by applying next rules:

The handler/action has a higher priority if it belongs to the current assembly (same assembly with request) and the other is not. If none of the objects belong to the current assembly, they can be considered equal. If both objects belong to the current assembly, they can't be compared only by this criterion - compare by next rule;
The handler/action has a higher priority if it belongs to the current/child request namespace and the other is not. If both objects belong to the current/child request namespace, they can be considered equal. If none of the objects belong to the current/child request namespace, they can't be compared by this criterion - compare by next rule;
The handler/action has a higher priority if it namespace is part of the current location (request namespace) and the other is not. If both objects are part of the current location, the closest has higher priority. If none of the objects are part of the current location, they can be considered equal.

Override handler/action

Create a new handler / action that inherits the handler / action that you want to override, and save it in accordance with the priority rules.

Exception handler vs Exception action

All actions will be performed for the thrown exception. However, only one handler can handle the thrown exception;
The exception will be re-thrown after all actions are completed. But the exception can be handled by the exception handler, and the result will be returned to the caller;
The actions execution process faster. Because the exception handler works like try / catch block with several catch and search handlers for all base exceptions separately;
Both support priority sorting;
Both support overriding.

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