Use custom async targets with NLog and dependency inject HttpClient

I am using a custom async target that writes logs to Splunk with NLog (5.0.1.) in my .NET 6 application. I want to leverage IHttpClientFactory in ASP.NET Core.

In my startup.cs, under ConfigureServices(), I register my new target and NLog configuration file.

Target.Register<SplunkTarget>("Splunk");
LogManager.LoadConfiguration("NLog.config");

When I try to inject the IHttpClientFactory into my target class's instructor like this:

private readonly IHttpClientFactory _clientFactory;
public SplunkTarget(IHttpClientFactory clientFactory)
{
     _clientFactory = clientFactory;
}

I'm unable to start the application because I get the error:

Exception thrown: 'System.MissingMethodException' in System.Private.CoreLib.dll: 'Cannot dynamically create an instance of type 'MyNamespace.SplunkTarget'. Reason: No parameterless constructor defined.'

How do I use the built-in .NET Core HttpFactory so that I don't have to manually create a Httpclient?

Solution

With NLog 5.0 then you can resolve dependencies during initialization:

public class MyCustomTarget : AsyncTaskTarget
{
    protected override void InitializeTarget()
    {
        // ResolveService will throw if dependency is not available,
        // and target-initialization will retry when dependency is available
        var clientFactory = ResolveService<IHttpClientFactory>();
    }
}

If the target must work at startup, then you must register the singleton-interface upfront:

NLog.LogManager.Setup().SetupExtensions(ext => ext.RegisterSingletonService<IHttpClientFactory>(clientFactory));

But you are also allowed to post-pone the registration until UseNLog() or AddNLog() are called, and the Microsoft ServiceProvider is registered as external dependency-injection-provider (All LogEvents generated until that point will be cached and released on target-initialization success).

P.S. Before NLog 5.0 then one could override the factory-method like this, and perform the dependency injection:

var defaultConstructor = ConfigurationItemFactory.Default.CreateInstance;

ConfigurationItemFactory.Default.CreateInstance.CreateInstance = type =>
{
   if (type == typeof(MyCustomTarget))
       return new MyCustomTarget(myCustomParameter);
   
   return defaultConstructor(type);
};