The current version of the Simple Injector Extensions contain RegisterDecorator extension methods that allow decorating instances with decorators.
In some scenarios it will be beneficial to delay building of the dependency graph. For decorators this means that instead of building up the entire dependency graph (decorator + wrapped instance), only the decorator should be created, injected with a factory that allows to create the wrapped at a later time. This question came up on Stackoverflow multiple times (see questions 10955077 and 10304023). Delaying the dependency graph is for instance needed when we need to execute the wrapped instance asynchronously, or when we want to execute the wrapped instance inside a lifetime scope.
The answers to these Stackoverflow questions contain a registration that can be generalized to:
container.ExpressionBuilt += (s, e) =>
var type = e.RegisteredServiceType;
if (type.IsGenericType &&
type.GetGenericTypeDefinition() == typeof(ICommandHandler<>))
var instanceCreator = Expression.Lambda(
var constructor =
e.Expression = Expression.New(constructor,
This however is some nasty code, especially for a problem that seems to be common. Besides, this code does not behave correctly when working with generic type constraints, generic type swapping, and other nasty generic type complexities. It would be good to allow the RegisterDecorator methods allow to delay building the dependency graph by injecting a Func<T> instead of just the dependency itself. Adding this feature is a non-breaking change.