Asp.Net Core: register implementation with multiple interfaces and lifestyle Singleton

Question

Considering the following interface and class definitions:

public interface IInterface1 { } public interface IInterface2 { } public class MyClass : IInterface1, IInterface2 { } 

is there any way to register one instance of MyClass with multiple interfaces like this:

... services.AddSingleton<IInterface1, IInterface2, MyClass>(); ... 

and resolve this single instance of MyClass with different interfaces like this:

IInterface1 interface1 = app.ApplicationServices.GetService<IInterface1>(); IInterface2 interface2 = app.ApplicationServices.GetService<IInterface2>(); 

Answer 1

The service collection by definition is a collection of ServiceDescriptors, which are pairs of service type and implementation type.

You can however get around this by creating your own provider function, something like this (thanks user7224827):

services.AddSingleton<IInterface1>(); services.AddSingleton<IInterface2>(x => x.GetService<IInterface1>()); 

More options below:

private static MyClass ClassInstance;  public void ConfigureServices(IServiceCollection services) {     ClassInstance = new MyClass();     services.AddSingleton<IInterface1>(provider => ClassInstance);     services.AddSingleton<IInterface2>(provider => ClassInstance); } 

Another way would be:

public void ConfigureServices(IServiceCollection services) {     ClassInstance = new MyClass();     services.AddSingleton<IInterface1>(ClassInstance);     services.AddSingleton<IInterface2>(ClassInstance); } 

Where we just provide the same instance.

Answer 2

You can wrap user7224827's answer to create a nice extension method matching your original desired API:

    public static class ServiceCollectionExt     {         public static void AddSingleton<I1, I2, T>(this IServiceCollection services)              where T : class, I1, I2             where I1 : class             where I2 : class         {             services.AddSingleton<I1, T>();             services.AddSingleton<I2, T>(x => (T) x.GetService<I1>());         }     }