How to register dependency injection with generic types? (.net core)

Question

I have an asp.net core web app with multiple parameters in appSettings.json file.

I didnt' want to have services having IOptions<MyObject> in the constructor.

I wanted MyObject in the constructor. So I found the following article: https://weblog.west-wind.com/posts/2017/dec/12/easy-configuration-binding-in-aspnet-core-revisited which is very interesting.

But I want to go further. I would like to create an extension method to generate the injection.

Here is what I would like to do:

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace Common.WebTools.Extensions
{
    public static class IServiceCollectionExtensions
    {
        public static IServiceCollection AddSingletonConfigurationObject<T>(this IServiceCollection services, 
            IConfiguration configuration,
            string appSettingsKey) where T:new()
        {   
            var obj2 = new T();
            configuration.Bind(appSettingsKey, obj);
            services.AddSingleton(obj2); //compilation failed
            return services;
        }
    }
}

And then in my ConfigureServices method I can call

services.AddSingletonConfigurationObject<Common.Tools.Configuration.GoogleAnalyticsConfiguration>(Configuration, "GoogleAnalytics");

But I Have a compliation error on this line:

services.AddSingleton(obj2);

Does somebody know how could I correct the error?

impossible cast from 'T' to 'System.Type' ->it doesn't use the good constructor — Rémy
– Rémy, Commented May 23, 2019 at 9:43
I am not sure but maybe it can't guess the type. Alternatively you can try serices.AddSingleton<T>(obj2) or services.AddSingleton(typeof(T), obj2) — Rabban
– Rabban, Commented May 23, 2019 at 9:49

Tony · Accepted Answer · 2019-05-23 10:48:37Z

106

You can use services.AddScoped to use only 1 instance in the scope request. So in general improvement compare to AddTransient

services.AddScoped(typeof(IGenericRepository<>), typeof(GenericRepository<>));

So my interface and class will look like this

public interface IGenericRepository<T> where T : class

public class GenericRepository<T> : IGenericRepository<T> where T : class

answered May 23, 2019 at 10:48

Tony

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4 Comments

AMG Over a year ago

So how would the target for the injected instance actually look?

AMG Over a year ago

Is it even possible with constructor injecton since it requires a type argument? private readonly IGenericRepository<???> _repository;

mRizvandi Over a year ago

private readonly IGenericRepository<T> GenericRepository; ....... in the constructor parameters use ....IGenericRepository<T> genericRepository; and in body of constructor set the property .... GenericRepository = genericRepository. you have to use it in generic class, with same T inheritance.

Patrick Szalapski Over a year ago

Any idea why this idea can't work with AddTransient?

Nkosi · Accepted Answer · 2019-05-23 11:13:00Z

3

As stated in the comments you should set the generic constraint to where T: class in order to satisfy the constraint of the AddSingleton call.

Reference AddSingleton(IServiceCollection, TService)

Additionally you can bind the object graph using ConfigurationBinder.Get<T>

Reference Configuration in ASP.NET Core : Bind to an object graph

ConfigurationBinder.Get<T> binds and returns the specified type. Get<T> is more convenient than using Bind.

like in the following example

public static class IServiceCollectionExtensions {
    public static IServiceCollection AddSingletonConfigurationObject<TService>(
        this IServiceCollection services, 
        IConfiguration configuration,
        string appSettingsKey) where TService: class
    {   

        var setting = configuration.GetSection(appSettingsKey).Get<TService>();
        services.AddSingleton(setting);
        return services;
    }
}

edited May 23, 2019 at 11:13

answered May 23, 2019 at 10:54

Nkosi

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1 Comment

palapapa Over a year ago

"You should set the generic constraint to where T: class in order to satisfy the constraint of the AddSingleton call." Why? The constraint of AddSingleton is just where TService : class where TImplementation : class, TService, which doesn't limit the type parameter of TService and TImplementation, right?

granadaCoder · Accepted Answer · 2021-10-04 18:17:27Z

0

I was able to use this style to encapsulate (as an extension method)

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;

namespace MyProject.DependencyInjection
{
    public static class MyEncapsulatedIocRegistration
    {
        public static IServiceCollection AddTypeT<T>(this IServiceCollection services) where T : class
        {
            services.AddSingleton<IMyThingInterface<T>, MyThingConcrete<T>>();

            return services;
        }
    }
}

answered Oct 4, 2021 at 18:17

granadaCoder

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