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StructureMap API to replace registration from registry

Harish Hareendra Babu

Harish Hareendra Babu

— 1 min read

StructureMap allows you to replace an already registered type mapping by clearing all previous registrations for the type and adding new one.

The example in the documentation clears all previous registration for the type within a registry. What if you want to replace the registration from the Container after mapping a type through a Registry?

One use case for this capability that I ran into recently was while writing a test case. I wanted to make use of the StructureMap’s Registry, but replace one of the type registration with an in-memory implementation. Assumptions made following the example in the documentation however did not produce the desired outcome.

Suppose you had the following classes and Registry.

public class InMemoryWidget : IWidget { }
public class Widget : IWidget { }

public class WidgetRegistry : Registry
{
    public WidgetRegistry()
    {
        For<IWidget>().Use<Widget>();
    }
}

Notice the difference in the registry compared to the example from StructureMap documentation.

Now, to replace the IWidget registration to use InMemoryWidget in the test, you might assume the following code to do the job.

var container = new Container(cfg =>
{
    cfg.AddRegistry<WidgetRegistry>();
    cfg.For<IWidget>().ClearAll().Use<InMemoryWidget>();
});

This doesn’t work as expected. If we printed out what the container contains using WhatDoIHave(), you will still see 2 registrations.

TestContext.Out.WriteLine(container.WhatDoIHave(pluginType: typeof(IWidget)));

Outputs…

================================================================================
PluginType     Namespace   Lifecycle     Description                 Name
--------------------------------------------------------------------------------
IWidget        Example     Transient   Example.Widget               (Default)
                           Transient   Example.InMemoryWidget
================================================================================

Notice how the first one (Widget) is the default. Because, it was added first. The one mapped after clearing all previous registration for IWidget from our test is appended to the list of registrations.

The reason for this I gathered is, because the previous registration was done via a Registry. And clearing registrations for all types of IWidget ignores the ones added via Registry. The previous approach will only clear registrations applied directly to the Container.

In order to clear the registration from the registry in this situation…

var container = new Container(cfg =>
{
  var registry = new WidgetRegistry();
  registry.For<IWidget>().ClearAll().Use<InMemoryWidget>();
  cfg.AddRegistry(registry);
});

You clear the registration from the instance of the registry and pass the instance to the container configuration’s AddRegistry() method. We make use of the AddRegistry overload that accepts an object instead of the type.

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