Named Instances and a Default Instance in StructureMap?

Question

In my StructureMap bootstrapping code I'm using a custom convention to scan assemblies and add interface/implementation pairs to the object graph as named instances. Essentially I have some logic which checks configuration settings and drills down to this statement depending on various conditions:

registry.For(interfaceType).Use(type)
    .Named(implementationName);

This adds all of the named instances well enough. However, I'd also like to add a default instance in the event that an instance name is not specified. However, the default instance isn't always the last one added to the graph. Sometimes other named instances are added afterward during the scanning. It would seem, though, that whichever instance is added last, regardless of whether or not it's named, is always the default.

I've tried various combinations of the fluent API, including:

registry.For(interfaceType).Add(type);

or:

registry.For(interfaceType).Use(type);

Even some of the ones marked as deprecated. But the resulting behavior is always that the last one is the default. So if the order of adding implementations is something like this:

1. For the Logger interface use the Log4Net implementation named "Log4Net"
2. For the Logger interface use the Log4Net implementation by default
3. For the Logger interface use the Mock implementation named "Mock"

The resulting behavior is that the "Mock" implementation is used as the default when no name is specified. Debugging into AllInstances in the container, I see in the following order:

1. An instance of the Log4Net logger named "Log4Net"
2. An instance of the Log4Net logger with a GUID for a name (like any other default instance, as far as I can tell)
3. An instance of the Mock logger named "Mock"

Debugging into a logging statement when called from the container without an instance name, however, results in the Mock implementation being used.

Is there a way to add a default instance to the object graph while still being able to add named instances afterward?

Answer 1

The Add method will add instances (if you need to add named instances or add multiple instances for to use with collections/enumerations). If no explicit default is registered (using the Use method), the last instance added will become the default instance. The Use method is intended for setting the default instance. If you invoke Use multiple times, the last instance registered will become default.

In order to set a default instance and then register further named instances you should be able to do it like this:

registry.For(typeof(Logger)).Use(typeof(Log4Net)).Named("Log4Net");
registry.For(typeof(Logger)).Add(typeof(Mock)).Named("Mock");

This will make the Log4Net instance the default and also accessible as a named instance. The Mock instance will be available as a named instance.