fix: void requests

Author: GerardSmit

src/Directory.Build.props

@@ -1,8 +1,8 @@
 <Project>
 
     <PropertyGroup>
-        <Version>1.0.0-alpha.18</Version>
-        <PackageVersion>1.0.0-alpha.18</PackageVersion>
+        <Version>1.0.0-alpha.19</Version>
+        <PackageVersion>1.0.0-alpha.19</PackageVersion>
         <Authors>Zapto</Authors>
         <RepositoryUrl>https://github.com/zapto-dev/Mediator</RepositoryUrl>
         <Copyright>Copyright © 2025 Zapto</Copyright>

src/Mediator.DependencyInjection/Generic/Handlers/GenericRequestHandler.cs

@@ -168,7 +168,7 @@ public async ValueTask Handle(IServiceProvider provider, TRequest request, Cance
         foreach (var registration in _enumerable)
         {
             if (registration.RequestType != requestType ||
-                registration.ResponseType != null)
+                registration.ResponseType != typeof(Unit))
             {
                 continue;
             }

src/Mediator.DependencyInjection/Generic/MediatorBuilder.RequestHandler.cs

@@ -18,6 +18,10 @@ public IMediatorBuilder AddRequestHandler(
             _services.Add(new ServiceDescriptor(handlerType, handlerType, GetLifetime(scope)));
             _services.AddSingleton(new GenericRequestRegistration(requestType, responseType, handlerType));
         }
+        else if (responseType == typeof(Unit))
+        {
+            _services.Add(new ServiceDescriptor(typeof(IRequestHandler<>).MakeGenericType(requestType), handlerType, GetLifetime(scope)));
+        }
         else
         {
             _services.Add(new ServiceDescriptor(typeof(IRequestHandler<,>).MakeGenericType(requestType, responseType), handlerType, GetLifetime(scope)));
@@ -39,7 +43,7 @@ public IMediatorBuilder AddRequestHandler(Type handlerType, RegistrationScope sc
         {
             var args = type.GetGenericArguments();
             var requestType = type.GetGenericArguments()[0];
-            var responseType = args.Length > 1 ? args[1] : null;
+            var responseType = args.Length > 1 ? args[1] : typeof(Unit);
 
             if (requestType.IsGenericType)
             {

tests/Mediator.DependencyInjection.Tests/RequestTest.cs

@@ -154,4 +154,46 @@ public async Task TestVoidRequest()
         _ = handler.Received()
             .Handle(Arg.Any<IServiceProvider>(), Arg.Any<VoidRequest>(), Arg.Any<CancellationToken>());
     }
+
+    private abstract class BaseRequestHandler<T> : IRequestHandler<T, int>
+        where T : IRequest<int>
+    {
+        public ValueTask<int> Handle(IServiceProvider provider, T request, CancellationToken cancellationToken) => new(1);
+    }
+
+    private class RequestHandler : BaseRequestHandler<Request>;
+
+    [Fact]
+    public async Task TestBaseRequestHandler()
+    {
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(b => b.AddRequestHandler(typeof(RequestHandler)))
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        var result = await mediator.Send(new Request());
+
+        Assert.Equal(1, result);
+    }
+
+    private abstract class BaseVoidRequestHandler<T> : IRequestHandler<T>
+        where T : IRequest
+    {
+        public ValueTask Handle(IServiceProvider provider, T request, CancellationToken cancellationToken) => default;
+    }
+
+    private class BaseVoidRequestHandler : BaseVoidRequestHandler<VoidRequest>;
+
+    [Fact]
+    public async Task TestBaseVoidRequestHandler()
+    {
+        var serviceProvider = new ServiceCollection()
+            .AddMediator(b => b.AddRequestHandler(typeof(BaseVoidRequestHandler)))
+            .BuildServiceProvider();
+
+        var mediator = serviceProvider.GetRequiredService<IMediator>();
+
+        await mediator.Send(new VoidRequest());
+    }
 }