Instantiating objects with .NET Core's DI container

Question

I'm using an IServiceCollection to create a list of required services for my objects. Now I want to instantiate an object and have the DI container resolve the dependencies for that object

Example

// In my services config.
services
    .AddTransient<IMyService, MyServiceImpl>();

// the object I want to create.
class SomeObject
{
    public SomeObject(IMyService service)
    {
        ...
    }
}

How to I get the DI container to create an object of type SomeObject, with the dependecies injected? (presumably this is what it does for controllers?)

Note: I do not want to store SomeObject in the services collection, I just want to be able to do something like this...

SomeObject obj = startup.ServiceProvider.Resolve<SomeObject>();

... Rationale: I don't have to add all of my controllers to the service container, so I don't see why I would have to add SomeObject to it either!?

@Nkosi In the Main method of a console app...something like SomeObject x = startup.ServiceProvider.ResolveDependenciesFor<SomeObject>(); — Matthew Layton
– Matthew Layton, Commented Oct 30, 2016 at 22:39
relevant article: msdn.microsoft.com/en-us/magazine/mt707534.aspx — Nkosi
– Nkosi, Commented Oct 30, 2016 at 22:42
Rationale: I don't have to add all of my controllers to the service container, so I don't see why I would have to add SomeObject to it either The framework is doing that for you. it is using a convention and registers all controllers for you — Nkosi
– Nkosi, Commented Oct 30, 2016 at 22:46

kiripk · Accepted Answer · 2018-11-14 12:29:02Z

21

As stated in the comments to the marked answer, you can use ActivatorUtilities.CreateInstance method. This functionality already exists in .NET Core (since version 1.0, I believe).

See: https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.activatorutilities.createinstance

answered Nov 14, 2018 at 12:29

kiripk

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Comments

Matthew Layton · Accepted Answer · 2016-10-30 23:25:01Z

6

It's a little rough, but this works

public static class ServiceProviderExtensions
    {
        public static TResult CreateInstance<TResult>(this IServiceProvider provider) where TResult : class
        {
            ConstructorInfo constructor = typeof(TResult).GetConstructors()[0];

            if(constructor != null)
            {
                object[] args = constructor
                    .GetParameters()
                    .Select(o => o.ParameterType)
                    .Select(o => provider.GetService(o))
                    .ToArray();

                return Activator.CreateInstance(typeof(TResult), args) as TResult;
            }

            return null;
        }
    }

answered Oct 30, 2016 at 23:25

Matthew Layton

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10 Comments

Alex Wiese Over a year ago

How is this using the IServiceProvider to instantiate the object (with all dependencies injected)?

Matthew Layton Over a year ago

@alexw .Select(o => provider.GetService(o))

Matthew Layton Over a year ago

@alexw basic principle, provided there is one constructor, iterate over it's parameter types, resovle them using the service provider and then create an instance using the results.

Matthew Layton Over a year ago

@flodin It's been a long time since I wrote this, but yes I think this functionality does already exist. The DI stuff baked into .NET Core implements the Service Locator pattern. This should lead you in the right direction... joonasw.net/view/aspnet-core-di-deep-dive

kahoona Over a year ago

oke but in the .NET Core way they say it's : var instance = (IPipe)ActivatorUtilities.CreateInstance(serviceProvider, pipeType); . But then you still need a ServiceProvider object :-(. I can't understand why they made it so difficult to just instantiate a class with DI in a self written class.

|

David Ali · Accepted Answer · 2021-07-27 15:26:57Z

1

Extension method:

public static class Extensions
{
    public static T BuildObject<T>(this IServiceProvider serviceProvider, params object[] parameters)
        => ActivatorUtilities.CreateInstance<T>(serviceProvider, parameters);
}

Usage:

[HttpGet]
public IEnumerable<WeatherForecast> Get()
{
    var ss = HttpContext.RequestServices.BuildObject<SomeService>();
}

answered Jul 27, 2021 at 15:26

David Ali

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