MediatR

Simple mediator implementation in .NET

In-process messaging with no dependencies.

Supports request/response, commands, queries, notifications and events, synchronous and async with intelligent dispatching via C# generic variance.

Examples in the wiki.

Installing MediatR

You should install MediatR with NuGet:

Install-Package MediatR

Or via the .NET Core command line interface:

dotnet add package MediatR

Either commands, from Package Manager Console or .NET Core CLI, will download and install MediatR and all required dependencies.

Using Contracts-Only Package

To reference only the contracts for MediatR, which includes:

• IRequest (including generic variants)
• INotification
• IStreamRequest

Add a package reference to MediatR.Contracts

This package is useful in scenarios where your MediatR contracts are in a separate assembly/project from handlers. Example scenarios include:

• API contracts
• GRPC contracts
• Blazor

Registering with IServiceCollection

MediatR supports Microsoft.Extensions.DependencyInjection.Abstractions directly. To register various MediatR services and handlers:

services.AddMediatR(cfg => cfg.RegisterServicesFromAssemblyContaining<Startup>());

or with an assembly:

services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(Startup).Assembly));

This registers:

• IMediator as transient
• ISender as transient
• IPublisher as transient
• 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

This also registers open generic implementations for:

• INotificationHandler<>
• IRequestExceptionHandler<,,>
• IRequestExceptionAction<,>

To register behaviors, stream behaviors, pre/post processors:

services.AddMediatR(cfg => {
    cfg.RegisterServicesFromAssembly(typeof(Startup).Assembly);
    cfg.AddBehavior<PingPongBehavior>();
    cfg.AddStreamBehavior<PingPongStreamBehavior>();
    cfg.AddRequestPreProcessor<PingPreProcessor>();
    cfg.AddRequestPostProcessor<PingPongPostProcessor>();
    cfg.AddOpenBehavior(typeof(GenericBehavior<,>));
    });

With additional methods for open generics and overloads for explicit service types.

Open behaviors with nested or extra type parameters

Open behaviors and processors whose type parameters map one-to-one onto the interface, like GenericBehavior<TRequest, TResponse>, are registered as open generics and closed by the container. Others can't be closed by the container positionally, such as a nested response or a type parameter that only appears in a constraint:

public class ListBehavior<TRequest, TItem> : IPipelineBehavior<TRequest, List<TItem>>
    where TRequest : IRequest<List<TItem>> { ... }

public class TransactionBehavior<TRequest, TResponse, TDbContext> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : ITransactionalCommand<TDbContext>
    where TDbContext : IDbContext { ... }

By default, MediatR closes these itself: it scans the registered assemblies for matching request types and registers a closed behavior for each one, filling extra type parameters from the constraints. Requests outside the scanned assemblies don't get the behavior.

If you use a container that closes open generics by matching the service type, such as DryIoc, LightInject or Autofac, and your requests come from assemblies that aren't scanned, you can leave closing to the container instead. Don't use this with the Microsoft or Stashbox containers: they close open generics positionally, and reject an open generic with a different number of type parameters than the interface when the service provider is built.

services.AddMediatR(cfg => {
    cfg.RegisterServicesFromAssembly(typeof(Startup).Assembly);
    cfg.OpenBehaviorClosingStrategy = OpenBehaviorClosingStrategy.CloseInContainer;
    cfg.AddOpenBehavior(typeof(ListBehavior<,>));
    });

Behaviors with a type parameter that only appears in a constraint, like TDbContext in TransactionBehavior<,,>, can't be closed by any container and are always closed from scanned requests.

Setting the license key

You can set the license key when registering MediatR:

services.AddMediatR(cfg => 
{
    cfg.LicenseKey = "<license key here>";
})

Or if not using Microsoft.Extensions.DependencyInjection:

Mediator.LicenseKey = "<license key here>";

Tip

The license key does not need to be set on client applications (such as Blazor WASM). Turn off the license warning by configuring logging in your logging start configuration: builder.Logging.AddFilter("LuckyPennySoftware.MediatR.License", LogLevel.None);

Auto-discovery via environment variables

If no license key is set in code, MediatR looks for one in environment variables. This is convenient for containerized and cloud environments, and for enterprises that share a single key across many services without code changes:

• MEDIATR_LICENSE_KEY – the MediatR-specific license key.
• LUCKYPENNY_LICENSE_KEY – a shared key usable across Lucky Penny products (for example, AutoMapper reads the same variable). Because it is shared, the key must be for a license that includes MediatR (a Bundle or MediatR edition); an AutoMapper-only license will not validate here.

The license key is resolved in the following order of precedence, using the first value found:

1. An explicit value set in code (cfg.LicenseKey or Mediator.LicenseKey).
2. The MEDIATR_LICENSE_KEY environment variable.
3. The LUCKYPENNY_LICENSE_KEY environment variable.

No code change is required when using an environment variable—just register MediatR as usual without setting the license key.

You can register for your license key at MediatR.io

Renewal grace period

License keys include a 30-day grace period after the end of their term. When your subscription renews, you receive a new license key, and the previous key keeps logging only a WARNING (not an ERROR) for 30 days after its term ends. This gives you time to deploy the renewed key. Your current license key is always available on your account page.