Override autofac registration with plugin

Question

I have an IFoo service implemented by DefaultFoo, and I've registered it as such in my autofac container.

Now I would like to allow for an alternative implementation of IFoo to be implemented in a plugin assembly, which can be dropped in a "plugins" folder. How do I configure autofac to prefer this alternative implementation if it is present?

score 21 · Accepted Answer · edited Jun 24 '17 at 17:46

21

If you register some interface implementations, Autofac will use the latest registration. Other registrations will be overridden. In your case, Autofac will use the plugin registration, if plugin exists and register own IFoo service implementation.

If more than one component exposes the same service, Autofac will use the last registered component as the default provider of that service.

See Default Registrations

edited Jun 24 '17 at 17:46

t3chb0t

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answered Apr 02 '13 at 03:18

Memoizer

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Is this expected behaviour with Autofac, or just coincidental? Do you have any references? – Rob Bell Jan 30 '15 at 11:56
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This suggests it is expected: http://docs.autofac.org/en/latest/register/registration.html#default-registrations – Rob Bell Jan 30 '15 at 12:27
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Note that other registrations are not overriden! Autofac will just use the last registered component, but the previous components can still be resolved (Resolve> for example) – Bastiaan Linders Jan 17 '18 at 14:41

Wim Coenen · Answer 2 · 2013-04-03T12:52:39.903

As stated by Memoizer, the latest registration overrides earlier ones. I ended up with something like this:

// gather plugin assemblies
string applicationPath = Path.GetDirectoryName(
    Assembly.GetEntryAssembly().Location);
string pluginsPath = Path.Combine(applicationPath, "plugins");
Assembly[] pluginAssemblies = 
    Directory.EnumerateFiles(pluginsPath, "*.dll")
    .Select(path => Assembly.LoadFile(path))
    .ToArray();

// register types
var builder = new ContainerBuilder();
builder.Register<IFoo>(context => new DefaultFoo());
builder.RegisterAssemblyTypes(pluginAssemblies)
    .Where(type => type.IsAssignableTo<IFoo>())
    .As<IFoo>();

// test which IFoo implementation is selected
var container = builder.Build();
IFoo foo = container.Resolve<IFoo>();
Console.WriteLine(foo.GetType().FullName);

This is super useful for testing because it allows to override _default_ registrations with their _test_ counterparts. — t3chb0t, Jun 24 '17 at 17:41

Override autofac registration with plugin

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