Immediate.Handlers

Immediate.Handlers is an implementation of the mediator pattern in .NET using source-generation. All pipeline behaviors are determined and the call-tree built at compile-time; meaning that all dependencies are enforced via compile-time safety checks. Behaviors and dependencies are obtained via DI at runtime based on compile-time determined dependencies.

Examples

• Minimal Api: Normal

Installing Immediate.Handlers

You can install Immediate.Handlers with NuGet:

Install-Package Immediate.Handlers

Or via the .NET Core command line interface:

dotnet add package Immediate.Handlers

Either commands, from Package Manager Console or .NET Core CLI, will download and install Immediate.Handlers.

Using Immediate.Handlers

Creating Handlers

Create a Handler by adding the following code:

[Handler]
public sealed partial class GetUsersQuery(
	UsersService usersService
)
{
    public record Query;

    private ValueTask<IEnumerable<User>> HandleAsync(
        Query _,
        CancellationToken token
	)
    {
        return usersService.GetUsers();
    }
}

This will automatically create a new class, GetUsersQuery.Handler, which encapsulates the following:

• attaching any behaviors defined for all queries in the assembly
• using a class to receive any DI services, such as UsersService

Any consumer can now do the following:

public class Consumer(GetUsersQuery.Handler handler)
{
	public async Task Consumer(CancellationToken token)
	{
		var response = await handler.HandleAsync(new(), token);
		// do something with response
	}
}

For Command handlers, use a ValueTask, and Immediate.Handlers will insert a return type of ValueTuple to your handler automatically.

[Handler]
public sealed partial class CreateUserCommand()
    UsersService usersService
)
{
    public record Command(string Email);

    private async ValueTask HandleAsync( 
        Command command,
        CancellationToken token
	)
    {
        await usersService.CreateUser(command.Email);
    }
}

In case your project layout does not allow direct for references between consumer and handler, the handler will also be registered as an IHandler<TRequest, Response>.

public class Consumer(IHandler<Query, IEnumerable<User>> handler)
{
	public async Task Consumer(CancellationToken token)
	{
		var response = await handler.HandleAsync(new(), token);
		// do something with response
	}
}

Creating Behaviors

Create a behavior by implementing the Immediate.Handlers.Shared.Behaviors<,> class, as so:

public sealed class LoggingBehavior<TRequest, TResponse>(ILogger<LoggingBehavior<TRequest, TResponse>> logger)
	: Behavior<TRequest, TResponse>
{
	public override async ValueTask<TResponse> HandleAsync(TRequest request, CancellationToken cancellationToken)
	{
		logger.LogInformation("LoggingBehavior.Enter");
		var response = await Next(request, cancellationToken);
		logger.LogInformation("LoggingBehavior.Exit");
		return response;
	}
}

Using Behaviors

Once added to the pipeline, the behavior will be called as part of the pipeline to handle a request. They can be added to the pipeline one of three ways:

• Behaviors can be registered assembly-wide by using an [assembly: ] attribute, as shown here:

[assembly: Behaviors(
	typeof(LoggingBehavior<,>)
)]

• Behaviors can be applied on an individual handler using:

[Handler]
[Behavior(
	typeof(LoggingBehavior<,>)
)]
public static class GetUsersQuery
{
	// ..
}

• Common behavior pipelines can be defined by applying a [Behaviors] attribute another attribute, as shown here:

[Behaviors(
  typeof(ValidationBehavior<,>), typeof(TransactionBehavior<,>)
)]
public sealed class DefaultBehaviorsAttribute : Attribute;

// usage
[Handler]
[DefaultBehaviors]
public sealed class GetUsersQuery
{
	// ..
}

Note: adding a [Behavior] attribute to a handler will disregard all assembly-wide behaviors for that handler, so any global behaviors necessary must be independently added to the handler override behaviors list.

Behavior Constraints

A constraint can be added to a behavior by using:

public sealed class LoggingBehavior<TRequest, TResponse>
	: Behavior<TRequest, TResponse>
	where TRequest : IRequestConstraint
	where TResponse : IResponseConstraint

When a pipeline is generated, all potential behaviors are evaluated against the request and response types, and if either type does not match a given constraint, the behavior is not added to the generated pipeline.

Registering with IServiceCollection

Immediate.Handlers supports Microsoft.Extensions.DependencyInjection.Abstractions directly.

Registering Handlers

In your Program.cs, add a call to services.AddXxxHandlers(), where Xxx is the shortened form of the project name.

• For a project named Web, it will be services.AddWebHandlers()
• For a project named Application.Web, it will be services.AddApplicationWebHandlers()

This registers all classes in the assembly marked with [Handler].

Registering Behaviors

In your Program.cs, add a call to services.AddXxxBehaviors(), where Xxx is the shortened form of the project name.

• For a project named Web, it will be services.AddWebBehaviors()
• For a project named Application.Web, it will be services.AddApplicationWebBehaviors()

This registers all behaviors referenced in any [Behaviors] attribute.

Using with Swashbuckle

For Swagger to work the JSON schema generated is required to have unique schemaId's. To achieve this, Swashbuckle uses class names as simple schemaId's. When using Immediate Handlers classes with a controller action inside, you might end up with Swashbuckle stating an error similar to this:

Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Failed to generate schema for type - MyApp.Api.DeleteUser+Command. See inner exception
System.InvalidOperationException: Can't use schemaId "$Command" for type "$MyApp.Api.DeleteUser+Command". The same schemaId is already used for type "$MyApp.Api.CreateUserCommand+Command"

This error indicates Swashbuckle is trying to use two classes named Command from two (or more) different Handlers in different namespaces.

To fix this, you have to define the following options in your SwaggerGen configuration:

builder.Services.AddSwaggerGen( options =>
{
    options.CustomSchemaIds(x => x.FullName?.Replace("+", ".", StringComparison.Ordinal));
});

Performance Comparisons

For performance comparisons, check out https://github.com/ImmediatePlatform/MediatorBenchmarks.