Spring.net - how to choose implementation of interface in runtime ?

Question

Hi, in all examples of spring.net IoC i can see something like this:

interface IClass;

class ClassA : IClass;

class ClassB : IClass,

and then in config.xml file something like

[object id="IClass" type="ClassB, Spring.Net.Test" /]

but, i really need to do something like this: in config file there will be more implementations if interface:

[object id="IClass" type="ClassA, Blah" /]

[object id="IClass" type="ClassB, Blah" /]

and then, in runtime i choose from them, something like this:

IClass c = [get me all implementations of IClass, and choose the one with GetType().FullName == myVariableContainingFullTypeNameOfObjectIWant]

how can i do something like this please, i cant google anything for hours.... many thanks !

Answer 1

maybe you could try this:

[object id="Blah.ClassA" type="ClassA, Blah" /]

[object id="Blah.ClassB" type="ClassB, Blah" /]

IClass = (IClass) ApplicationContext.GetObject(myVariableContainingFullTypeNameOfObjectIWant);

Answer 2

I have done something similar before and took an approach very similar to the one Fabiano has suggested.

Sample config:

<object id="ClassAInstance" type="ClassA, Blah"> ... </object>

<object id="ClassBInstance" type="ClassB, Blah"> ... </object>

Now some generalized sample code using the WebApplicationContext:

        IApplicationContext context = new XmlApplicationContext(locations);
        IClass c = (IClass)context.GetObject(declarationId);

There are a couple of things to note:

1. Pass in the id of the declaration you wish to use not the Type name, so the variable declarationId would have the value of either "ClassAInstance" or "ClassBInstance".
2. The constructor to XmlApplicationContext (and WebApplicationContext) takes a parameter of an array of string values; the variable locations would be an array of configuration resources to search through to find the object with id declarationId. You cannot use a generic List here, it has to be an actual string array.

One interesting implication of point 2 above is that you actually control which configuration resources your ApplicationContext is aware of: when you call the GetObject() method, the ApplicationContext will only search for your object within the configuration resources given in the array locations[]. This means that rather than list all possible configurations within one file each with a unique id, you can have multiple configuration resources instead, each containing one object declaration and each with the same id:

Config1.xml: <object id="IClassInstance" type="ClassA, Blah"> ... </object>

Config2.xml: <object id="IClassInstance" type="ClassA, Blah"> ... </object>

But when instantiating the object, you can control which object is created not based on the declarationId, which in both cases will be "IClassInstance", but by changing the locations[] array to contain the configuration resource you want to use, in this case either Config1.xml or Config2.xml

Hope this is of use,

Andrew