Add test helper that registers a type with all its interfaces

src/Merq.Core/MessageBus.cs

@@ -394,7 +394,7 @@ static Type GetCommandType(IExecutable command)
                 var generic = descriptor.ServiceType.GetGenericTypeDefinition();
                 // Consider both void and non-void handlers
                 if (generic != typeof(IExecutableCommandHandler<>) &&
-                    generic != typeof(IExecutableCommandHandler<,>) && 
+                    generic != typeof(IExecutableCommandHandler<,>) &&
                     generic != typeof(ICommandHandler<>) &&
                     generic != typeof(ICommandHandler<,>) &&
                     generic != typeof(IAsyncCommandHandler<>) &&

src/Merq.Tests/ServiceCollectionExtensions.cs

@@ -0,0 +1,30 @@
+using System;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Merq;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddAll<TImplementation>(this IServiceCollection services)
+        where TImplementation : class, new()
+    {
+        AddAll(services, _ => new TImplementation());
+        return services;
+    }
+
+    public static IServiceCollection AddAll<TImplementation>(
+        this IServiceCollection services,
+        Func<IServiceProvider, TImplementation> implementationFactory)
+        where TImplementation : class
+    {
+        // Simulates what the DependencyInjection.Attribute does, by registering 
+        // the implementation with all the interfaces it implements.
+
+        services.AddSingleton(implementationFactory);
+
+        foreach (var interfaceType in typeof(TImplementation).GetInterfaces())
+            services.AddSingleton(interfaceType, s => s.GetRequiredService(typeof(TImplementation)));
+
+        return services;
+    }
+}