var services = new ServiceCollection();
foreach (var type in GetType().Assembly.DefinedTypes)
{
    var interfaces = type.GetInterfaces()
        .Where(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(ICommandHandler<,>))
        .ToArray();
    if (!interfaces.Any())
        continue;
    services.AddSingleton(type);
    foreach (var iCommandHandler in interfaces)
    {
        Assert.AreEqual(typeof(ICommandHandler<Command1, int>), iCommandHandler);
        services.AddSingleton(iCommandHandler, s => s.GetService(type));
    }
}

Assert.IsTrue(services.Any(d => d.ServiceType == typeof(ICommandHandler<Command1, int>)));
var serviceProvider = services.BuildServiceProvider();
var ex = Assert.ThrowsException<InvalidOperationException>(() =>
{
    // var command1Handler = serviceProvider.GetRequiredService(typeof(ICommandHandler<Command1, int>));  // error too!
    var command1Handler = serviceProvider.GetRequiredService<ICommandHandler<Command1, int>>();
});
Assert.AreEqual(
    "No service for type 'Test.ICommandHandler`2[Test.Command1,System.Int32]' has been registered.",
    ex.Message);
// Assert.AreEqual(1, command1Handler.Handle(new Command1()));

public interface ICommand<TResponse>
{

}

public interface ICommandHandler<in TCommand, TResponse> where TCommand : ICommand<TResponse>
{
    Task<TResponse> Handle(TCommand request);
}

public class Command1 : ICommand<int>
{

}

public class CommandsHandler : ICommandHandler<Command1, int>
{
    public Task<int> Handle(Command1 request)
    {
        Console.WriteLine("Command1 has been handled");
        return Task.FromResult(1);
    }
}