[Question] Can IMediatR be registered as Singleton?

I’m using a library that registers a type I define (that consumes IMediatr) as Singleton. Mediatr is registered in the ServiceCollection extensions as Scoped, so I’m getting an InvalidOperationException: Cannot consume scoped service 'MediatR.IMediator' from singleton 'Xyz.Abc' exception.

I can circumvent the ServiceCollection extensions and do my own registration if need be, just wanted some advice on lifetime.

FYI: This is for an ASP.NET Core web api style app.

Thanks!

[jbogard]

The general rule of thumb is singletons can only depend on other singletons, all the way down. This is true really for any DI container. If you don’t want this behavior, you have two options:

• Don’t make your service a singleton
• Resolve dynamically through a scoped container on-demand (inside a method or otherwise)

This is also true for any DI container that supports scoped/nested containers. It’s exactly what ASP.NET Core does per request - so in your case, your choices are a bit limited.

There’s nothing preventing you from registering IMediator as a singleton, HOWEVER, this forces every handler, and every handler’s dependencies, to be singleton.

You could also provide your own implementation of IMediator to do what the ASP.NET Core code does above - create a scope and resolve handlers from that scope. I don’t because I don’t want to couple the Mediator implementation to scopes, and I have the control of just registering all the dependencies as Transient.

[henkmollema]

You could create your own scope inside the singleton instance and retrieve instances from it. For example:

public MySingletonService(IServiceProvider serviceProvider)
{
    _serviceProvider = serviceProvider;
}

public void Foo()
{
    using (var scope = _serviceProvider.CreateScope())
    {
        var scopedProvider = scope.ServiceProvider;
    }
}