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Handler Selectors

Modified on 2010/11/17 19:42 by Neal Blomfield Categorized as BuildingBlocks


Handler selectors let you dynamically choose a component to satisfy the requested service and override Windsor's default behavior. This is particularly useful in multi-tenant applications.
Custom naming subsystem

Only DefaultNamingSubSystem works with selectors. All other implementations of INamingSubSystem provided out of the box with Windsor ignore selectors.

How it works

Selectors are types implementing IHandlerSelector interface which exposes two methods:

bool HasOpinionAbout(string key, Type service);

IHandler SelectHandler(string key, Type service, IHandler[] handlers);

The HasOpinionAbout method is called to ask the selector if it would like to select a handler for the given service (described by key or/and type). If the call returns true then SelectHandler will be called which should select a single handler from the given array. The selector can also return null in which case the next selector in turn will be asked.

The array contains all assignable handlers

Notice that the handlers array passed to the SelectHandler method contains all handlers that expose services assignable to service (which may mean, in case when service is not provided, all handlers in the entire container). This gives you ability to select handler that was not explicitly registered with the service. Also take note Windsor does not do any filtering of the handlers it gives you, so some of them may not be resolvable (may have some required dependencies missing), which is something you should take into the account when implementing your selectors.

The selection algorithm

When more than one selector is registered, Windsor will call HasOpinionAbout on each of them in the order in which they were registered until one returns true. Windsor will then invoke the SelectHandler method, and if non-null handler is returned, that handler will be used to satisfy the service. If the call to SelectHandler returns null Windsor will continue asking the remaining selectors following the same algorithm. If none of the selectors return a handler Windsor will fall back to its default behavior.

How to use it

You attach selectors to the container using the following method:

container.Kernel.AddHandlerSelector(new MySelector());

External resources

(05 Oct, 2008) blogpost by Ayende, discussing the feature

ScrewTurn Wiki version Some of the icons created by FamFamFam.