Calling Generic Methods from Non-Generic Code in .Net

Somewhat often (or at least it feels that way this week) I’ll run into the need to call a method with a generic type argument from code that isn’t generic. To make that concrete, here’s an example from Marten. The main IDocumentSession service has a method called Store() that directs Marten to persist one or more documents of the same type. That method has this signature:

void Store<T>(params T[] entities);

That method would typically be used like this:

using (var session = store.OpenSession())
{
    // The generic constraint for "Team" is inferred from the usage
    session.Store(new Team { Name = "Warriors" });
    session.Store(new Team { Name = "Spurs" });
    session.Store(new Team { Name = "Thunder" });

    session.SaveChanges();
}

Great, and easy enough (I hope), but Marten also has this method where folks can add a heterogeneous mix of any kind of document types all at once:

void StoreObjects(IEnumerable<object> documents);

Internally, that method groups the documents by type, then delegates to the property Store<T>() method for each document type — and that’s where this post comes into play.

(Re-)Introducing Baseline

Baseline is a library available on Nuget that provides oodles of little helper extension methods on common .Net types and very basic utilities that I use in almost all my projects, both OSS and at work. Baseline is an improved subset of what was long ago FubuCore (FubuCore was huge, and it also spawned Oakton), but somewhat adapted to .Net Core.

I wanted to call this library “spackle” because it fills in usability gaps in the .Net base class library, but Jason Bock beat me to it with his Spackle library of extension methods. Since I expected this library to be used as a foundational piece from within basically all the projects in the JasperFx suite, I chose the name “Baseline” which I thought conveniently enough described its purpose and also because there’s an important throughway near the titular Jasper called “Baseline”. I don’t know for sure that it’s the basis for the name, but the Battle of Carthage in the very early days of the US Civil War started where this road is today.

Crossing the Non-Generic to Generic Divide with Baseline

Back to the Marten StoreObjects(object[]) calling Store<T>(T[]) problem. Baseline has a helper extension method called CloseAndBuildAs<T>() method I frequently use to solve this problem. It’s unfortunately a little tedious, but first design a non-generic interface that will wrap the calls to Store<T>() like this:

internal interface IHandler
{
    void Store(IDocumentSession session, IEnumerable<object> objects);
}

And a concrete, open generic type that implements IHandler:

internal class Handler<T>: IHandler
{
    public void Store(IDocumentSession session, IEnumerable<object> objects)
    {
        // Delegate to the Store<T>() method
        session.Store(objects.OfType<T>().ToArray());
    }
}

Now, the StoreObjects() method looks like this:

public void StoreObjects(IEnumerable<object> documents)
{
    assertNotDisposed();

    var documentsGroupedByType = documents
        .Where(x => x != null)
        .GroupBy(x => x.GetType());

    foreach (var group in documentsGroupedByType)
    {
        // Build the right handler for the group type
        var handler = typeof(Handler<>).CloseAndBuildAs<IHandler>(group.Key);
        handler.Store(this, group);
    }
}

The CloseAndBuildAs<T>() method above does a couple things behind the scenes:

1. It creates a closed type for the proper Handler<T> based on the type arguments passed into the method
2. Uses Activator.CreateInstance() to build the concrete type
3. Casts that object to the interface supplied as a generic argument to the CloseAndBuildAs<T>() method

The method shown above is here in GitHub. It’s not shown, but there are some extra overloads to also pass in constructor arguments to the concrete types being built.