how-properties-are-injected.md

How properties are injected

Property injection of dependencies is designed to be done during component activation when a component is created. The responsibility of determining which properties are used for injection is fulfilled by default through PropertiesDependenciesModelInspector - a IContributeComponentModelConstruction implementation which uses all the following criteria to determine if a property represents a dependency:

• Has 'public' accessible setter
• Is an instance property
• If ComponentModel.InspectionBehavior is set to PropertiesInspectionBehavior.DeclaredOnly, is not inherited
• Does not have parameters
• Is not annotated with the Castle.Core.DoNotWireAttribute attribute

If a property meets all these criteria, a dependency model is created for it, and this is then resolved when the component dependencies are resolved during activation.

Customization

Property injection can be controlled via a number of mechanisms, the most common are listed below.

DoNotWireAttribute

A simple method to exempt a property from being chosen for injection is to add a Castle.Core.DoNotWireAttribute to the property declaration. This method is best for the occasional exception.

public class MyComponent
{
   public IFoo Foo { get; set; }

   [DoNotWire]
   public IBar MyOtherDependency { get; set; }
}

In the example above, the MyOtherDependency has the attribute, and Windsor will ignore it, never trying to set it when creating instances of this component.

Encapsulization

Making a property setter protected or internal will prevent it from chosen as a dependency property. This can be useful in designing a service which is also protected against arbitrary setting of properties after resolution by clients, making a tight, well-focused service interface.

Removing the PropertiesDependenciesModelInspector

A DefaultKernel has a ComponentModelBuilder which contains a PropertiesDependenciesModelInspector in the Contributors list. It can be removed and then no property injection will be performed on any component. To remove it, use code similar to the following:

// We don't want to inject properties, only ctors
var propInjector = Kernel.ComponentModelBuilder
                         .Contributors
                         .OfType<PropertiesDependenciesModelInspector>()
                         .Single();
Kernel.ComponentModelBuilder.RemoveContributor(propInjector);

Replacing the PropertiesDependenciesModelInspector

PropertiesDependenciesModelInspector can be subclassed, and the method InspectProperties can be overridden to accomodate specialized logic to support your needed scenario.