using simpleinjector to register opengeneric

Question

I have the below classes:

public interface IDbCommandHandler<in TCommand, out TOutput> 
    where TCommand : IDbCommand 
{
    TOutput Handle(TCommand command);
}

public class SubIdentifierItemCreateCommand<TItemType, TDefaultValues> 
    : BaseDbCommand
    where TItemType: TDefaultValues
{

}

public class SubIdentifierItemCreateCommandHandler<TItemType, TDefaultValues>
    : BaseDbCommandHandler<SubIdentifierItemCreateCommand<TItemType, TDefaultValues>, TItemType>,
    IDbCommandHandler<SubIdentifierItemCreateCommand<TItemType, TDefaultValues>, TItemType>
    where TItemType: class, TDefaultValues, IItemForGenericItemByIdentifierRetriever , new()
{

}

I need to register the SubIdentifierItemCreateCommandHandler as singleton-open-generic, to handle any requests for services of type IDbCommandHandler<SubIdentifierItemCreateCommand<,>,>.

Is this possible? I've tried in various ways and I always get an error.

_container.RegisterSingleOpenGeneric(
    typeof(IDbCommandHandler<,>),
    typeof(SubIdentifierItemCreateCommandHandler<,>));

_container.RegisterOpenGeneric(
    typeof(IDbCommandHandler<,>),
    typeof(SubIdentifierItemCreateCommandHandler<,>));

// this one is throws a compile-time error, that you cannot 
// use partial open types.
_container.RegisterManyForOpenGeneric(
    typeof(IDbCommandHandler<SubIdentifierItemCreateCommand<,>,>),
    typeof(SubIdentifierItemCreateCommandHandler<,>)); 

I want to be able to call the below and work:

var item = _container.GetInstance<
    IDbCommandHandler<
        SubIdentifierItemCreateCommand<
            SectionData, 
            ISectionDataDefaultValues>, 
        SectionData>>();

Answer 1

Unfortunately, you stumbled upon a bug in the framework. Simple Injector 1.6.1 does not correctly handle the "where TItemType: TDefaultValues" constraint correctly.

The solution is simple, migrate to Simple Injector 2.0 (via NuGet).

If you for what ever reason can't switch to Simple Injector 2.0, you can register a ResolveUnregisteredType event for the registration of the type to work around the bug in the 1.6.1 release:

container.ResolveUnregisteredType += (sender, e) =>
{
    var serviceType = e.UnregisteredServiceType;

    if (serviceType.IsGenericType &&
        serviceType.GetGenericTypeDefinition() == typeof(IDbCommandHandler<,>))
    {
        var commandArg = serviceType.GetGenericArguments()[0];
        var outputArg = serviceType.GetGenericArguments()[1];

        if (commandArg.IsGenericType &&
            commandArg.GetGenericTypeDefinition() == 
                typeof(SubIdentifierItemCreateCommand<,>))
        {
            var itemTypeArgument = commandArg.GetGenericArguments()[0];
            var defaultValuesArgument = commandArg.GetGenericArguments()[1];

            if (itemTypeArgument != outputArg)
            {
                return;
            }

            Type typeToRegister;

            try
            {
                typeToRegister =
                    typeof(SubIdentifierItemCreateCommandHandler<,>)
                    .MakeGenericType(itemTypeArgument.GetGenericArguments());
            }
            catch (ArgumentException)
            {
                // Thrown by MakeGenericType when the type constraints 
                // do not match. In this case, we don't have to register
                // anything and can bail out.
                return;
            }

            object singleInstance = container.GetInstance(typeToRegister);

            // Register the instance as singleton.
            e.Register(() => singleInstance);
        }
    }
};

I know, it is ugly, but at least there's a work around ;-)