C# .NET CORE WEB.API — Inject Multiple Instance of some Interface

Let’s assume that we’re writing one ISomeService interface and 3 different classes, SomeServiceA, SomeServiceBand SomeServiceC implementing the interface.

public interface ISomeService{
string Title { get; }
}
-=--=--=--=--=--=--=--=--=--=--=--=--=--=-
public class SomeServiceA: ISomeService
{
 public string Title { get { return "A"; } }
}
-=--=--=--=--=--=--=--=--=--=--=--=--=--=-
public class SomeServiceB: ISomeService{
public string Title { get { return "B"; } }
}
-=--=--=--=--=--=--=--=--=--=--=--=--=--=-
public class SomeServiceC: ISomeService{
public string Title { get { return "C"; } }
}

👎 Instead of registering them like this 👎

public void ConfigureServices(IServiceCollection services)
{ 
services.AddTransient<ISomeService, SomeServiceA>(); services.AddTransient<ISomeService, SomeServiceB>(); services.AddTransient<ISomeService, SomeServiceC>();
}

😎 We are gonna extend Assembly like this 😎

public static class AssemblyExtension
{
public static IEnumerable<Type> GetTypesFrom<T>(this Assembly assembly)
{
List<Type> types = new List<Type>();
foreach (var type in assembly.DefinedTypes)
{
if (typeof(T).IsAssignableFrom(type) && typeof(T) != type)
{
types.Add(type);
}
}
return types;
}
}

The main idea is that if in the future we add another class that will implement the ISomeService interface, we will not have to register it in the services 😀

• typeof(T) != type validate that we “skip” the interface :)
• From Microsoft —

IsAssignableFrom - Determines whether an instance of a specified type can be assigned to a variable of the current type.

And in the startup we gonna use it like this -

Assembly.GetEntryAssembly().GetTypesFrom<ISomeService>()
.ForEach((t) =>{
services.AddTransient(typeof(ISomeService), t);
});

(the ForEach extension for IEnumerable) —

public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
foreach (T element in source){
action(element);
}
}

In the controller —

private readonly IEnumerable<ISomeService> _services;
.
.
.
[HttpGet]
[Route(“{serviceType}”)]
public IActionResult Get(string serviceType){
var concreteService = _services.FirstOrDefault(service => service.Title.ToLower() == serviceType.Trim().ToLower());
return Ok($”{concreteService.GetType().Name}”);
}

• [Optional] — we can extract the validation logic of serviceType to ActionFilter Instead of doing it at the Controller (If serviceType is invalid, or empty, etc., etc., etc.)

Good Luck 🤞,

Eran Hadad