How can I pass a runtime parameter as part of the dependency resolution?

Question

I need to be able to pass a connection string into some of my service implementations. I am doing this in the constructor. The connection string is configurable by user will be

Answer 1

To pass runtime parameter not known at the start of the application you have to use the factory pattern. You have two options here

1. factory method

    services.AddTransient>((provider) => 
    {
        return new Func( 
            (connectionString) => new NestedService(connectionString)
        );
    });
   

   and inject the factory method in your service instead of INestedService.

    public class RootService : IRootService
    {
        public INestedService NestedService { get; set; }
   
        public RootService(Func nestedServiceFactory)
        {
            NestedService = nestedServiceFactory("ConnectionStringHere");
        }
   
        public void DoSomething()
        {
            // implement
        }
    }
   

   or resolve it per call

    public class RootService : IRootService
    {
        public Func NestedServiceFactory { get; set; }
   
        public RootService(Func nestedServiceFactory)
        {
            NestedServiceFactory = nestedServiceFactory;
        }
   
        public void DoSomething(string connectionString)
        {
            var nestedService = nestedServiceFactory(connectionString);
   
            // implement
        }
    }
   

2. factory class

    public class RootServiceFactory : IRootServiceFactory 
    {
        // in case you need other dependencies, that can be resolved by DI
        private readonly IServiceCollection services;
   
        public RootServiceCollection(IServiceCollection services)
        {
            this.services = services;
        }
   
        public CreateInstance(string connectionString) 
        {
            // instantiate service that needs runtime parameter
            var nestedService = new NestedService(connectionString);
   
            // resolve another service that doesn't need runtime parameter
            var otherDependency = services.GetService()
   
            // pass both into the RootService constructor and return it
            return new RootService(otherDependency, nestedDependency);
        }
    }
   

   and inject IRootServiceFactory instead of your IRootService.

    IRootService rootService = rootServiceFactory.CreateInstance(connectionString);