手动解析InstancePerRequest类型时未收到Autofac相同的注入实例

Question

I'm trying to have a transaction ID which will be used for logging throughout the request. Trasaction ID is a property in AuditContext, which in turn will be a singleton per request. I've the below code in Global.asax.cs

builder.RegisterType<AuditContext>().As<IAuditContext>().InstancePerRequest();
....
GlobalContainer = builder.Build();
config.DependencyResolver = new AutofacWebApiDependencyResolver(GlobalContainer);

Transaction ID is set in a base api class.

public BaseApiController(IAuditContext auditContext, IDispatcher dispatcher = null)
{  
    auditContext.TransactionID = Guid.NewGuid().ToString();
}

AuditContext object with the correct transaction ID is injected, in below constructor injection usage.

public class SapEventLogger : ISapEventLogger
{        
    private IAuditContext auditContext;        

    public SapEventLogger(IAuditContext auditContext)
    {
        this.auditContext = auditContext; //this auditContext object has correct transaction ID
    }

}

In case of any exception I want to retrieve the transaction ID and log it. But when I try to manually resolve, I get a new AuditContext object in which the transaction ID is null.

public class ServiceExceptionHandler : ExceptionHandler
{
    public override void Handle(ExceptionHandlerContext context)
    {
        using (var scope = GlobalContainer.BeginLifetimeScope(Autofac.Core.Lifetime.MatchingScopeLifetimeTags.RequestLifetimeScopeTag))
        {
            var auditContext = scope.Resolve<FDP.Services.Common.IAuditContext>();
            transactionID = auditContext.TransactionID; //Transaction ID is null here
        }
    }
}

Not sure why a new object is created when resolving AuditContext manually. Am I missing something?

Answer 1

With the BeginLifetimeScope call you're manually creating a second request lifetime rather than using the existing request lifetime. Two request lifetimes equals two instances.

The solution to this is outlined in the Autofac docs in more detail but the short version is to get the request lifetime from the request message in the handler context:

// Get the request lifetime scope so you can resolve services.
var requestScope = context.Request.GetDependencyScope();

// Resolve the service you want to use.
var auditContext = requestScope.GetService(typeof(IAuditContext)) as IAuditContext;

// Do the rest of the work in the filter.
auditContext.DoWork();