Optional object reference

问题

I have a problem with my spring.net configuration:

<object id="O1">
    <constructor-arg ref="Dep1"/>
    <constructor-arg ref="Dep2"/>
</object>

This is part of my generic configuration used by all applications. Some applications define Dep2 in their configuration and some don't. Can I make second constructor arg return null (instead of reporting error) when specific application doesn't define Dep2?
I would like to solve this without overriding O1 definition in app specific configuration.

Thanks.

回答1:

You can create an IFactoryObject that returns null and configure it in your "specific application" config file. See this related question: How do I configure a NULL object in Spring.Net.

Furthermore, if Dep2 is an optional dependency (e.g it can be null or it has a sensible default) than it is probably better to define it as a property and use property injection.

Edit

I expected this to work, but it actually doesn't, because an IFactoryObject that returns null is treated as an error by the spring container:

From the api docs on IFactoryObject.GetObject()

If this method is being called in the context of an enclosing IoC container and returns , the IoC container will consider this factory object as not being fully initialized and throw a corresponding (and most probably fatal) exception.

Classes:

public class MyClass
{
    public MyOtherClass Prop { get; set; }

    public MyClass(MyOtherClass ref1)
    {
    }
}

public class MyOtherClass
{
}

public class NullFactoryObject : IFactoryObject
{
    public object GetObject()
    {
        return null;
    }

    public Type ObjectType
    {
        get { return typeof(MyOtherClass); }
    }

    public bool IsSingleton
    {
        get { return true; }
    }
}

in configfile1.xml:

<object id="MyObject" type="q9292066_null_object_reference.MyClass, q9292066_null_object_reference">
  <constructor-arg name="ref1" ref="ref1" />
</object>

in configfile2.xml:

<object id="ref1" 
        type="q9292066_null_object_reference.NullFactoryObject, q9292066_null_object_reference" />

来源：https://stackoverflow.com/questions/9292066/optional-object-reference

标签

dependency-injection

spring.net