问题
I am using Mediatr to handle messages from a queue. I can get a simple example to work. However I have run into problems when I try to inject an object into my handler
public class MessageCommandHandler : IRequestHandler<MessageCommand, bool>
{
private IMyDependency myDependency;
public MessageCommandHandler(IMyDependency myDependency)
{
this.myDependency = myDependency;
}
public Task<bool> Handle(MessageCommand request, CancellationToken cancellationToken)
{
return Task.FromResult(true);
}
}
This only works when I register IMyDependency as a transient scope, however when I register it as scoped lifetime it fails with the error
Cannot resolve 'MediatR.IRequestHandler`2[MyNamespace.MessageCommand,System.Boolean]' from root provider because it requires scoped service 'MyNamespace.IMyDependency'
I need to be able to inject dependencies with scoped lifetime. Has anyone got a solution for this.
I am using the .NET Core dependency injection framework. It is setup as follows
services.AddHostedService<QueueConsumer>();
services.AddScoped<IMyDependency, MyDependency>();
services.AddMediatR(new Assembly[] { Assembly.GetExecutingAssembly() });
Any ideas?
回答1:
Any time you use a dependency with a Scoped lifetime, you will need to use it inside a pre-created scope. In the case of MVC this would happen automatically behind the scenes but if you're using direct from your own code, say via a console application or something, you will need to create the scope yourself.
This can be done by injecting an instance of IServiceScopeFactory
and then using this factory to create a scope and then retrieve the dependency from that scope e.g.
public class MessageCommandHandler : IRequestHandler<MessageCommand, bool>
{
private IServiceScopeFactory _serviceScopeFactory;
public MessageCommandHandler(IServiceScopeFactory serviceScopeFactory)
{
_serviceScopeFactory = serviceScopeFactory;
}
public Task<bool> Handle(MessageCommand request, CancellationToken cancellationToken)
{
using (var scope = _serviceScopeFactory.CreateScope())
{
var scopedServices = scope.ServiceProvider;
var myDependency = scopedServices.GetRequiredService<IMyDependency>();
return Task.FromResult(true);
}
}
}
However (and note that the code above is untested), in my own systems I would almost always create the scope around whatever is sending the mediator request in which case any Scoped dependencies will still be injected automatically at this scope e.g.
... // some other calling class / Main method etc..
using (var scope = _serviceScopeFactory.CreateScope())
var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();
mediator.Send(new MessageCommand());
}
来源:https://stackoverflow.com/questions/57008673/mediatr-scope-problems