ServiceStack IAppSettings was not ready and would result NULL reference exception if used in constructor

It seems like the IAppSettings implementation was not ready from IoC in the constructor.

Before I go into details, I’ve read similar problems:

• ServiceStack doesn't auto-wire and register AppSettings
• Instantiation of POCO objects with ServiceStack's IAppSettings is not working

Both were answered by @mythz that he was not able to reproduce it.

From the Doc

"ServiceStack made AppSettings a first-class property, which defaults to looking at .NET’s App/Web.config’s.": https://docs.servicestack.net/appsettings#first-class-appsettings

And there is default IoC registration already in Funq to give you AppSettings when you ask for IAppSettings:

What I have

All my codes are in the repo: https://github.com/davidliang2008/MvcWithServiceStack

The demo app is just an ASP.NET MVC app (.NET 4.8) that built using the template, the simplest you can get, with ServiceStack (5.12.0) installed:

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        ...
        new AppHost().Init();
    }
}

public class AppHost : AppHostBase
{
    public AppHost() : base("MvcWithServiceStack", typeof(ServiceBase).Assembly) { }

    public override void Configure(Container container)
    {
        SetConfig(new HostConfig
        {
            HandlerFactoryPath = "api";
        }

        ControllerBuilder.Current.SetControllerFactory(new FunqControllerFactory(container));
    }
}

Then I have a base class for the ServiceStack Service, and a HelloService just to demo:

public abstract class ServiceBase : Service { }

public class HelloService : ServiceBase
{
    public IAppSettings AppSettings { get; set; }

    public object Get(HelloRequest request)
    {
        return new HelloResponse
        {
            Result = $"Hello, { request.Name }! Your custom value is { AppSettings.Get<string>("custom") }."
        };
    }
}

[Route("/hello/{name}")]
public class HelloRequest : IReturn<HelloResponse>
{
    public string Name { get; set; }
}

public class HelloResponse
{
    public string Result { get; set; }
}

What works

When you’re not using IAppSettings in the constructor, whether in the HelloService or its base class ServiceBase, everything works out fine.

When you clone the project to your local, if you navigate to /api/hello/{your-name}, you will see its response would be able to get the custom value from web.config:

What doesn’t work

When you’re trying to get the IAppSettings and initialize something else with some app setting values in the constructor – whether it’s in the child class or the base class, IAppSettings will fail to get the implementation from IoC, and result a NULL reference exception:

public abstract class ServiceBase : Service
{
    public IAppSettings AppSettings { get; set; }

    public ServiceBase()
    {
        // AppSettings would be NULL
        var test = AppSettings.Get<string>("custom");
    }
}

OR

public class HelloService : ServiceBase
{
    public HelloService()
    {
        // AppSettings would be NULL
        var test = AppSettings.Get<string>("custom");
    }
}

>Solution :

You cannot use any property dependency in the constructor since the properties can only be injected after the class is created and the constructor is run.

You’ll only be able to access it in the Constructor by using constructor injection, e.g:

public class HelloService : ServiceBase
{
    public HelloService(IAppSettings appSettings)
    {
        var test = appSettings.Get<string>("custom");
    }
}

Or accessing the dependency via the singleton:

public class HelloService : ServiceBase
{
    public HelloService()
    {
        var test = HostContext.AppSettings.Get<string>("custom");
    }
}